RegisterNativeAllocation: avoid an extraneous blocking gc.

If another thread already ran a blocking gc since
RegisterNativeAllocation was entered, there's no need to run another
blocking gc before exiting RegisterNativeAllocation.

Bug: 29156652
Bug: 32576211
Test: 004-NativeAllocations run test.
Change-Id: Ie89652760deaa24b70adb07227a9918059da46c7
1 file changed