Convert art gtests to Android.bp

This splits the compilation and running of the art gtests into two
separate locations.  The tests are now compiled in multiple Android.bp
modules in each directory.  art.go collects the installed locations of
each test and exports it as make variables.  art/build/Android.gtest.mk
converts the list into the rules to run the tests.

This has a few changes in behavior:
  - The rules to build tests are now always defined, and will build as
    part of mmma art or make checkbuild.
  - Host tests are no longer installed into out/host/linux-x86/bin, they
    are in out/host/linux-x86/nativetest[64]/<module name>/<test name>
  - Target tests are now in
    /data/nativetest[64]/art/<arch>/<module name>/<test name>

Test: mmma -j art
Test: m -j test-art-host
Test: m -j test-art-target
Change-Id: Iabcd99d43890e6b693688422b07a283c3226a496