Fixes #4712: Ensure CoordinatorLayout Behaviors are not renamed/remov…

…ed during binary optimizations (#4731)

## Explanation
Fixes #4712

Proguard was renaming CoordinatorLayout behaviors and removing
implementation constructors, where both can be referenced via reflection
in CoordinatorLayout. We haven't run into this issue before since it
seems the new bottom navigation component introduced in #4482 is the
first time we actually started using CoordinatorLayout behaviors.

Note that this is inherently difficult to test as it requires verifying
runtime functionality in a Proguard-optimized build. Hence, it's only
testable with end-to-end tests (and our own end-to-end tests currently
are manual tests) so no new unit/integration tests are being added.

Note also that this PR will be cherry-picked for the upcoming beta MR2
release.

## 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
This doesn't seem particularly valuable to fill out here--the video
would just show the options menu opening as it should (and does for
developer builds). I've verified it works locally, and #4712 will
require reverifying once the PR is merged,

config/proguard/androidx-proguard-rules.pro

@@ -36,6 +36,14 @@
 -keep class * extends androidx.room.RoomDatabase
 -keep @androidx.room.Entity class *
 
+# CoordinatorLayout uses reflection to load Behavior subclasses (see #4712 for context), so they
+# can't be renamed.
+-keep class * extends androidx.coordinatorlayout.widget.CoordinatorLayout$Behavior
+-keepclassmembers class * extends androidx.coordinatorlayout.widget.CoordinatorLayout$Behavior {
+    # Constructors may be referenced via reflection, so make sure they don't get removed.
+    <init>(...);
+}
+
 # A strange unknown issue that arises within a Room class (it seems an actual dependency is missing
 # within Room).
 -dontwarn androidx.room.paging.LimitOffsetDataSource