android build: Allow Gradle to use even more memory

We are pretty certain that the upstream commit
flutter/flutter@df114fbe9
was the exact one that caused the increased RAM usage for the Android
build.

The upstream change remove a default that previously enabled stripping
the debug symbols from the engine artifacts. This comes with intentional
regressions, as noted in: flutter/flutter#162675

Since the regression was expected, there is no action to be taken
upstream.

However, we haven't found an effective way to rewrite the build script in
a way that this is mitigated without needing to raise the limit. For the
investigation details, see CZO discussion:
  https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/Gradle.20out.20of.20memory

Since a previous bump to 3 GiB, the issue has been mitigated, but it
would still occur occasionally. Adding another 1 GiB hopefully should
address the flakes.

Author: PIG208

android/gradle.properties

@@ -1,4 +1,4 @@
-org.gradle.jvmargs=-Xmx3072M
+org.gradle.jvmargs=-Xmx1536M
 android.useAndroidX=true
 android.enableJetifier=true