Increase gradle heap size

The update to Gradle 5.1.1 decreased the default heap size to 512MB and our
build runs into Out-of-Memory errors.

Setting the gradle flags to higher values instead.

See https://developer.android.com/studio/releases/gradle-plugin#3-4-0

PiperOrigin-RevId: 247908526

gradle.properties

@@ -3,3 +3,4 @@ android.useAndroidX=true
 android.enableJetifier=true
 android.enableUnitTestBinaryResources=true
 buildDir=buildout
+org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=512m