Don't use ANDROID_BUILD_TOP in common_test.mk

The environment variable has been deprecated. Instead just use PPID.

Test: mma -j40 test-art-host-gtest

Change-Id: I229ae92a972f3cd28ff9de6d186b798de3b328c2
diff --git a/build/Android.common_test.mk b/build/Android.common_test.mk
index 0f29f29..3d1f434 100644
--- a/build/Android.common_test.mk
+++ b/build/Android.common_test.mk
@@ -23,10 +23,10 @@
 ifneq ($(TMPDIR),)
 ART_HOST_TEST_DIR := $(TMPDIR)/test-art-$(shell echo $$PPID)
 else
-# Use a BSD checksum calculated from ANDROID_BUILD_TOP and USER as one of the
-# path components for the test output. This should allow us to run tests from multiple
-# repositories at the same time.
-ART_HOST_TEST_DIR := /tmp/test-art-$(shell echo ${ANDROID_BUILD_TOP}-${USER} | sum | cut -d ' ' -f1)
+# Use a BSD checksum calculated from PPID and USER as one of the path
+# components for the test output. This should allow us to run tests from
+# multiple repositories at the same time.
+ART_HOST_TEST_DIR := /tmp/test-art-$(shell echo $$PPID-${USER} | sum | cut -d ' ' -f1)
 endif
 
 # List of known broken tests that we won't attempt to execute. The test name must be the full