android build: Allow Gradle to use even more memory, 4 GiB

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 causes the engine artifacts to contain debug
symbols, which is intentional (as it enables putting debug symbols in
the app's bundle artifact) but makes the artifacts about 8x larger.
That causes known regressions in CPU and memory use at build time:
  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
still happens some of the time: at least twice in the past day or so.
Add another 1 GiB to see if that addresses the flakes.

android/gradle.properties

@@ -1,4 +1,4 @@
-org.gradle.jvmargs=-Xmx3072M
+org.gradle.jvmargs=-Xmx4G
 android.useAndroidX=true
 android.enableJetifier=true