RegisterNativeAllocation: Avoid case of double blocking gc.

If multiple threads call RegisterNativeAllocation causing the blocking
watermark to be exceeded while a background GC is in progress, then
two back-to-back blocking GCs would be performed when a single
blocking GC is sufficient. For example:

1. Thread A RegisterNativeAllocation triggers background GC1
2. Thread A RegisterNativeAllocation triggers blocking GC2
3. Thread A's GC2 waits for GC1 to complete.
4. Thread B RegisterNativeAllocation sees GC2 in progress and waits for
it to complete before triggering a second blocking GC3.
5. GC1 completes.

Because thread B's RegisterNativeAllocation was called before GC1
completed and GC2 had not begun for real, thread B can simply wait for
GC2 to start and finish before returning, rather than waiting for GC2 to
start and finish and then doing a second blocking GC.

This change fixes the behavior so that only a single blocking GC is
performed in this case.

Bug: 36851903
Test: 004-NativeAllocations run test.
Change-Id: I1e178b9ee7bb68703bdc9a09b1041a982de8b2ce