Split the allocation path into 'instrumented' and 'uninstrumented'

ones.

The instrumented path is equivalent to the existing allocation path
that checks for three instrumentation mechanisms (the debugger
allocation tracking, the runtime allocation stats collection, and
valgrind) for every allocation. The uinstrumented path does not
perform these checks. We use the uninstrumented path by default and
enable the instrumented path only when any of the three mechanisms is
enabled. The uninstrumented version of Heap::AllocObject() is inlined.

This change improves the Ritz MemAllocTest by ~4% on Nexus 4 and ~3%
on Host/x86.

Bug: 9986565
Change-Id: I3e68dfff6789d77bbdcea98457b694e1b5fcef5f