Improve the allocation speed.

- Improves the Ritz MemAllocTest benchmark result by ~500 ms (or ~5%) on Nexus 4.
- Move the memset() call that zeroes the allocated memory out of the lock region.
- De-virtualize/Inline the allocation call chains into Heap::AllocObject().
- Turn Heap::measure_allocation_time_ into a static const variable.
- Surround the VerifyObject() call with kIsDebugBuild.

Bug: 9986565
Change-Id: Ib70b6d051a80ec329788b30256565561f031da2a
9 files changed