Merge changes from topic "nscobie-hwui_unit_tests"

* changes:
  Tweak GraphicsStats#findRootPath to accept various valid locations
  Enable hwui_unit_tests in presubmit for libs/hwui
  Fix GraphicsStats#findRootPath and disable CacheManager#trimMemory tests
  Change hwui_unit_tests (and benchmarks) to push to non-root writable dir