Remove unused dependencies from native-activity.

These dependencies were not actually used but were in contradiction with
the `android:hasCode=false` in the manifest. Remove the pointless
dependencies, but leave some comments explaining to anyone that copies
from here that they need to update `app:hasCode` if they add any DEX to
their app.

Fixes #929.

native-activity/app/build.gradle

@@ -2,6 +2,10 @@ plugins {
     id 'com.android.application'
 }
 
+// Caution: If you add any Java or Kotlin code to this project, or if you add
+// any dependencies which themselves use Java/Kotlin, you'll need to remove
+// `android:hasCode` from main/AndroidManifest.xml.
+
 android {
     compileSdk 33
     ndkVersion '25.1.8937393'
@@ -30,9 +34,3 @@ android {
     }
     namespace 'com.example.native_activity'
 }
-
-dependencies {
-    implementation fileTree(dir: 'libs', include: ['*.jar'])
-    implementation 'androidx.appcompat:appcompat:1.5.1'
-    implementation 'androidx.constraintlayout:constraintlayout:2.1.4'
-}

native-activity/app/src/main/AndroidManifest.xml

@@ -4,7 +4,13 @@
     android:versionCode="1"
           android:versionName="1.0">
 
-  <!-- This .apk has no Java code itself, so set hasCode to false. -->
+  <!--
+  This .apk has no Java/Kotlin code, so set hasCode to false.
+
+  If you copy from this sample and later add Java/Kotlin code, or add a
+  dependency on a library that does (such as androidx), be sure to set
+  `android:hasCode` to `true` (or just remove it, since that's the default).
+  -->
   <application
       android:allowBackup="false"
       android:fullBackupContent="false"