ART: Add GC callbacks

Add start and end of pause callbacks.

Fix setup and missing functions in test 907.

Bug: 31684633
Test: m test-art-host
Test: m test-art-host-run-test-908-gc-start-finish
Change-Id: I1d8872ef9cd6914de7fb033cd873f8d5cb48ea17
diff --git a/test/Android.bp b/test/Android.bp
index c303616..aaa1343 100644
--- a/test/Android.bp
+++ b/test/Android.bp
@@ -251,6 +251,7 @@
         "905-object-free/tracking_free.cc",
         "906-iterate-heap/iterate_heap.cc",
         "907-get-loaded-classes/get_loaded_classes.cc",
+        "908-gc-start-finish/gc_callbacks.cc",
     ],
     shared_libs: [
         "libbase",