Give the option of having an existing adb.

Test: test-art-target-gtest
Change-Id: Ib8d1246a0f55e16f67af6b7f54b5b4fc8c53b0de
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 316ce64..d024e77 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -93,6 +93,6 @@
 endif
 
 ADB_EXECUTABLE := $(HOST_OUT_EXECUTABLES)/adb
-ADB := $(ADB_EXECUTABLE)
+ADB ?= $(ADB_EXECUTABLE)
 
 endif # ART_ANDROID_COMMON_MK