Bump memory usage for flutter gallery (#132968)

Fixes https://github.com/flutter/flutter/issues/131006
Flakes related to out of memory in gradle when running tests. 

Opted not to add enforcement for gradle.properties in https://github.com/flutter/flutter/blob/master/dev/bots/analyze.dart because the memory usage has been stable for 6 years and not every example needs more memory.

dev/integration_tests/flutter_gallery/android/gradle.properties

-org.gradle.jvmargs=-Xmx1536M
+org.gradle.jvmargs=-Xmx4G
 android.useAndroidX=true
 android.enableJetifier=true