Bump up errorprone heap size

ag/19500070 causes errorprone to fail with out of memory errors.
Also in aosp/2215048 the regular javac heap sized was increased
to 4096MB, so the errorprone heap size wasn't any larger anymore.

Bug: 240473481
Test: m RUN_ERROR_PRONE=true SystemUIGoogleScreenshotTestsLib on internal master
Change-Id: Ie6bdce9f2e7620c076213f4c3313960fd537967b