android build: Enable core library desugaring, needed by flutter_local_notifications

This applies an Android setup step called for
by package:flutter_local_notifications:
  https://github.com/MaikuB/flutter_local_notifications/tree/d565daab613052fecaf723df865101b46ee5a26a/flutter_local_notifications#gradle-setup

namely enabling core-library desugaring:
  https://developer.android.com/studio/write/java8-support#library-desugaring

We seem to have been fine without that step so far.  But after the
upcoming upgrade to AGP 8.5.2 (from 8.1.4), the build starts giving
an error demanding this change:

Execution failed for task ':app:checkReleaseAarMetadata'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
   > An issue was found when checking AAR metadata:

       1.  Dependency ':flutter_local_notifications' requires core library desugaring to be enabled
           for :app.

           See https://developer.android.com/studio/write/java8-support.html for more
           details.

[greg: expanded commit message, added comment]

Author: rajveermalviya

android/app/build.gradle

@@ -28,6 +28,13 @@ android {
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
+
+        // The use of desugar_jdk_libs causes warning noise at build time:
+        //   https://github.com/zulip/zulip-flutter/pull/887#issuecomment-2287653388
+        //   https://issuetracker.google.com/issues/294273986
+        // TODO(#351): Try removing core-library desugaring once we've
+        //   removed flutter_local_notifications.
+        coreLibraryDesugaringEnabled true
     }
 
     kotlinOptions {
@@ -97,4 +104,5 @@ flutter {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlinVersion"
+    coreLibraryDesugaring "com.android.tools:desugar_jdk_libs:2.0.4"
 }