Fix race in AllocEntrypointsInstrumented

We were using the quick_alloc_entry_points_instrumentation_counter_,
this counter is updated before the threads are suspended. The
allocator could come out of a suspend point, see that threads are
supposedly still no instrumented, continue the allocation, then
suddenly quick_alloc_entry_points_instrumentation_counter_ becomes
1 and alloc_tracking_enabled_ becomes true resulting in a failing
DCHECK.

The fix is to add a boolean that is updated only when the threads
are suspended.

Bug: 27506909
Change-Id: Ic3c6716d2af9c00189ce9271f7a2825b03cdd8a0