ART: Fix GC pause reporting

CC doesn't use ScopedPause normally, but that was hidden by it using
it for a debug check. Add a parameter for reporting.

Add explicit marking of the pause in CC. Currently this will conservatively
approximate pause times, as the actual pause is hidden inside ThreadList.

Clean up run-test 908.

Bug: 31684633
Test: m ART_TEST_RUN_TEST_NDEBUG=true test-art-host-run-test-908-gc-start-finish
Change-Id: Ie48af52e1225783b06e2e088c5adf3ef42fb5f64