Fix #4645: "Continue Studying" texts keeps showing even when there is…

… no next revision card (#4743)

## Explanation
Fixes #4645 

This PR hides the "Continue Studying" text when there is only one
revision card in the list. It does so by adding a condition to the
visibility of this view inside the corresponding xml file. This PR is
backed up by unit tests.

## Essential Checklist
- [x] The PR title and explanation each start with "Fix #bugnum: " (If
this PR fixes part of an issue, prefix the title with "Fix part of
#bugnum: ...".)
- [x] Any changes to
[scripts/assets](https://github.com/oppia/oppia-android/tree/develop/scripts/assets)
files have their rationale included in the PR explanation.
- [x] The PR follows the [style
guide](https://github.com/oppia/oppia-android/wiki/Coding-style-guide).
- [x] The PR does not contain any unnecessary code changes from Android
Studio
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#undo-unnecessary-changes)).
- [x] The PR is made from a branch that's **not** called "develop" and
is up-to-date with "develop".
- [x] The PR is **assigned** to the appropriate reviewers
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#clarification-regarding-assignees-and-reviewers-section)).

## For UI-specific PRs only
If your PR includes UI-related changes, then:
- Add screenshots for portrait/landscape for both a tablet & phone of
the before & after UI changes
- For the screenshots above, include both English and pseudo-localized
(RTL) screenshots (see [RTL
guide](https://github.com/oppia/oppia-android/wiki/RTL-Guidelines))
- Add a video showing the full UX flow with a screen reader enabled (see
[accessibility
guide](https://github.com/oppia/oppia-android/wiki/Accessibility-(A11y)-Guide))
- Add a screenshot demonstrating that you ran affected Espresso tests
locally & that they're passing

Before

![image](https://user-images.githubusercontent.com/64526117/202563107-452cbbf0-aa14-43cb-91c6-6b256061851f.png)

After

![image](https://user-images.githubusercontent.com/64526117/202565577-00b2299e-f92f-4dcf-9f0a-30830641e191.png)

app/src/main/res/layout/revision_card_fragment.xml

@@ -49,6 +49,7 @@
           android:textColor="@color/color_def_black"
           android:textSize="20sp"
           android:textStyle="bold"
+          android:visibility="@{viewModel.subtopicListSize == 1 ? View.INVISIBLE : View.VISIBLE}"
           app:layout_constraintEnd_toEndOf="parent"
           app:layout_constraintStart_toStartOf="parent"
           app:layout_constraintTop_toTopOf="parent" />

app/src/sharedTest/java/org/oppia/android/app/topic/revisioncard/RevisionCardFragmentTest.kt

@@ -95,6 +95,7 @@ import org.oppia.android.domain.topic.FRACTIONS_TOPIC_ID
 import org.oppia.android.domain.topic.PrimeTopicAssetsControllerModule
 import org.oppia.android.domain.topic.SUBTOPIC_TOPIC_ID
 import org.oppia.android.domain.topic.SUBTOPIC_TOPIC_ID_2
+import org.oppia.android.domain.topic.TEST_TOPIC_ID_0
 import org.oppia.android.domain.translation.TranslationController
 import org.oppia.android.domain.workmanager.WorkManagerConfigurationModule
 import org.oppia.android.testing.BuildEnvironment
@@ -202,6 +203,39 @@ class RevisionCardFragmentTest {
     }
   }
 
+  @Test
+  fun testRevisionCard_openSubtopicWithOnlyOneSubtopicInList_checkContinueStudyingTextNotShown() {
+    launch<ExplorationActivity>(
+      createRevisionCardActivityIntent(
+        context,
+        profileId.internalId,
+        TEST_TOPIC_ID_0,
+        SUBTOPIC_TOPIC_ID,
+        1
+      )
+    ).use {
+      testCoroutineDispatchers.runCurrent()
+      onView(withId(R.id.continue_studying_text_view)).check(matches(not(isDisplayed())))
+    }
+  }
+
+  @Test
+  fun testRevisionCard_openSubtopicWithMultipleSubtopicsInList_checkContinueStudyingTextIsShown() {
+    launch<ExplorationActivity>(
+      createRevisionCardActivityIntent(
+        context,
+        profileId.internalId,
+        FRACTIONS_TOPIC_ID,
+        SUBTOPIC_TOPIC_ID,
+        FRACTIONS_SUBTOPIC_LIST_SIZE
+      )
+    ).use {
+      testCoroutineDispatchers.runCurrent()
+      onView(withId(R.id.continue_studying_text_view)).perform(scrollTo())
+      onView(withId(R.id.continue_studying_text_view)).check(matches(isDisplayed()))
+    }
+  }
+
   @Test
   fun testRevisionCardTest_openBottomSheet_selectHelpInOverflowMenu_opensHelpActivity() {
     launch<ExplorationActivity>(