Hold heap bitmap lock in Heap::GetObjectsAllocated

Fixes a race condition where add and remove space could cause a crash
when we iterated over the spaces.

TODO: Add a spaces lock or something to guard against this.

(cherry picked from commit a395c0a4)

Bug: 21031927

Change-Id: I7f0d558316f8e9d9f22ffd182e8666355bf50d47