Basic Jack integration.

First Step:
- flag (useJack) on BuildType to enable Jack.
- requires build tools 21.1.0 (not released)
- flag bypasses proguard/jacoco completely if used.
- works with apps and test apps.
- Probably disable lint since lint requires reading the generated
  .class files.

Change-Id: Id98b914309076171efb7d28d8618c08ba60fd925
24 files changed