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
diff --git a/build/Android.common.mk b/build/Android.common.mk
index a6a9f0f..316ce64 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -92,4 +92,7 @@
   2ND_ART_HOST_OUT_SHARED_LIBRARIES := $(2ND_HOST_OUT_SHARED_LIBRARIES)
 endif
 
+ADB_EXECUTABLE := $(HOST_OUT_EXECUTABLES)/adb
+ADB := $(ADB_EXECUTABLE)
+
 endif # ART_ANDROID_COMMON_MK