Bug 1786164 - Add more Gradle JVM memory flags. r=geckoview-reviewers,jonalmeida

See https://github.com/mozilla-mobile/fenix/pull/26542 for more discussion.
Also specify a lower maximum heap size more in line with other projects and
remove some redundant settings in other gradle.properties files.

Differential Revision: https://phabricator.services.mozilla.com/D155179

gradle.properties

 org.gradle.parallel=true
 org.gradle.daemon=true
-org.gradle.jvmargs=-Xmx32768M
+# Be careful when changing org.gradle.jvmargs below. It will clobber
+# any defaults set by Gradle: https://github.com/gradle/gradle/issues/19750
+org.gradle.jvmargs=-Xmx8g -Xms2g -XX:MaxMetaspaceSize=6g
 android.useAndroidX=true

mobile/android/gradle/dotgradle-offline/gradle.properties

 // Per https://docs.gradle.org/current/userguide/build_environment.html, this
 // overrides the gradle.properties in topsrcdir.
 org.gradle.daemon=false
-org.gradle.jvmargs=-Xmx32768M

mobile/android/gradle/dotgradle-online/gradle.properties

 // Per https://docs.gradle.org/current/userguide/build_environment.html, this
 // overrides the gradle.properties in topsrcdir.
 org.gradle.daemon=false
-org.gradle.jvmargs=-Xmx32768M