Clear empty unevac regions in ClearFromSpace

Clear empty unevac regions in ClearFromSpace

The motivation is to release RAM for empty unevac regions at the end
of the current GC instead of next GC.

Results on RitzPerf:
This optimization accounts for 2.33GB out of 53GB of freed bytes ~4%.
This also means that the average heap size is probably 4% smaller,
though it may not translate to RAM savings for actual apps.

Bug: 35800768
Bug: 12687968

Test: test-art-host
Change-Id: I38fa8dcda1aa6b439060ff170941b3a53022846f