Optionally dump ART crash information to logcat when running on Android.

This logging feature does not use debuggerd.  It is only
enabled when the environment variable ANDROID_ROOT is set
to something different than "/system".

Also, refactor runtime/runtime_android.cc and
runtime/runtime_linux.cc.

Test: Observe device logcat when dalvikvm crashes
Bug: 32466479
Change-Id: If59ce838fdb5b9bb6638b73fcd074b7065275c6c