Add ReadMapFileAsyncSafe function.

This is a function designed so that it doesn't do any allocations.
It's purpose is for testing allocation related code where doing an
allocation can disturb the test.

Added unit tests for the new code that disable memory allocations
before running.

Also, made the tests run in isolation mode since I added a bunch of
tests and this will keep the total runtime about the same as
before.

Bug: 137795072

Test: All unit tests pass.
Change-Id: I7645f1e9f88cdfbc57c8eebce80199b9cfae64c1