Compile with javac by default

javac is replacing jack as the default compiler for platform builds.

Bug: 36902714
Test: m -j checkbuild
Test: https://android-build.googleplex.com/builds/view-workplan?viewType=Table&workplanId=L50600000086240009&nodeType=Trybot
Test: sailfish-userdebug boots and basic interactions work
Test: art tests pass
Test: libcore tests pass
Change-Id: I1c3857f64eafa445ad1b9dd94eff85bef05f4649