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.gtest.mk b/build/Android.gtest.mk
index 2c073fe..20f20c9 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -461,14 +461,14 @@
 
 .PHONY: $$(gtest_rule)
 $$(gtest_rule): test-art-target-sync
-	$(hide) adb shell touch $$(PRIVATE_GTEST_WITNESS)
-	$(hide) adb shell rm $$(PRIVATE_GTEST_WITNESS)
-	$(hide) adb shell $$(PRIVATE_MAYBE_CHROOT_COMMAND) chmod 755 $$(PRIVATE_TARGET_EXE)
+	$(hide) $(ADB) shell touch $$(PRIVATE_GTEST_WITNESS)
+	$(hide) $(ADB) shell rm $$(PRIVATE_GTEST_WITNESS)
+	$(hide) $(ADB) shell $$(PRIVATE_MAYBE_CHROOT_COMMAND) chmod 755 $$(PRIVATE_TARGET_EXE)
 	$(hide) $$(call ART_TEST_SKIP,$$@) && \
-	  (adb shell "$$(PRIVATE_MAYBE_CHROOT_COMMAND) env $(GCOV_ENV) LD_LIBRARY_PATH=$(4) \
+	  ($(ADB) shell "$$(PRIVATE_MAYBE_CHROOT_COMMAND) env $(GCOV_ENV) LD_LIBRARY_PATH=$(4) \
 	       ANDROID_ROOT=$(ART_GTEST_TARGET_ANDROID_ROOT) $$(PRIVATE_TARGET_EXE) \
 	     && touch $$(PRIVATE_GTEST_WITNESS)" \
-	   && (adb pull $$(PRIVATE_GTEST_WITNESS) /tmp/ && $$(call ART_TEST_PASSED,$$@)) \
+	   && ($(ADB) pull $$(PRIVATE_GTEST_WITNESS) /tmp/ && $$(call ART_TEST_PASSED,$$@)) \
 	   || $$(call ART_TEST_FAILED,$$@))
 	$(hide) rm -f /tmp/$$@-$$$$PPID