Introduce GAE JSON endpoint & model definitions.

This pulls in the JSON-specific portions for the GAE Retrofit endpoint
configured in #4885 as part of a broader effort of splitting it into
multiple PRs.

scripts/src/java/org/oppia/android/scripts/gae/json/AndroidActivityEndpointApi.kt

@@ -0,0 +1,67 @@
+package org.oppia.android.scripts.gae.json
+
+import retrofit2.Call
+import retrofit2.http.GET
+import retrofit2.http.Query
+
+internal interface AndroidActivityEndpointApi {
+  @GET("android_data?activity_type=classroom")
+  fun fetchLatestClassroom(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeClassroom>>>
+
+  @GET("android_data?activity_type=exploration")
+  fun fetchLatestExploration(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeExploration>>>
+
+  @GET("android_data?activity_type=exploration")
+  fun fetchExplorationByVersion(
+    @Query("activities_data") request: AndroidActivityRequests.NonLocalized
+  ): Call<List<VersionedStructure<GaeExploration>>>
+
+  @GET("android_data?activity_type=story")
+  fun fetchLatestStory(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeStory>>>
+
+  @GET("android_data?activity_type=story")
+  fun fetchStoryByVersion(
+    @Query("activities_data") request: AndroidActivityRequests.NonLocalized
+  ): Call<List<VersionedStructure<GaeStory>>>
+
+  @GET("android_data?activity_type=skill")
+  fun fetchLatestConceptCard(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeSkill>>>
+
+  @GET("android_data?activity_type=skill")
+  fun fetchConceptCardByVersion(
+    @Query("activities_data") request: AndroidActivityRequests.NonLocalized
+  ): Call<List<VersionedStructure<GaeSkill>>>
+
+  @GET("android_data?activity_type=subtopic")
+  fun fetchLatestRevisionCard(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeSubtopicPage>>>
+
+  @GET("android_data?activity_type=subtopic")
+  fun fetchRevisionCardByVersion(
+    @Query("activities_data") request: AndroidActivityRequests.NonLocalized
+  ): Call<List<VersionedStructure<GaeSubtopicPage>>>
+
+  @GET("android_data?activity_type=learntopic")
+  fun fetchLatestTopic(
+    @Query("activities_data") request: AndroidActivityRequests.Latest
+  ): Call<List<VersionedStructure<GaeTopic>>>
+
+  @GET("android_data?activity_type=learntopic")
+  fun fetchTopicByVersion(
+    @Query("activities_data") request: AndroidActivityRequests.NonLocalized
+  ): Call<List<VersionedStructure<GaeTopic>>>
+
+  @GET("android_data?activity_type=exp_translations")
+  fun fetchExplorationTranslations(
+    @Query("activities_data") request: AndroidActivityRequests.Localized
+  ): Call<List<VersionedStructure<GaeEntityTranslations>>>
+}