Define $(ADB) in ART makefiles instead of hard-coding adb.

Follow-up after restrictions put in place by the build, see:
https://android.googlesource.com/platform/build/+/master/Changes.md#PATH_Tools

Test: test-art-target-sync, test-art-target-gtest
Change-Id: Ibd01be9c348ef1cc24d53d0d4c30c64eb1d9eac7
3 files changed