Move SwapBitmaps to ContinuousMemMapAllocSpace.

Moved the SwapBitmaps function to ContinuousMemMapAllocSpace since
the zygote space uses this function during full GC.

Fixed a place where we were casting a ZygoteSpace to a MallocSpace,
somehow this didn't cause any issues in non-debug builds.

Moved the CollectGarbage in PreZygoteFork before the lock to prevent
an occasional lock level violation caused by attempting to enqueue
java lang references with the a lock.

Bug: 12876255

Change-Id: I77439e46d5b26b37724bdcee3a0948410f1b0eb4