Speed up StepIfSignalHandler path.

The StepIfSignalHandler code reads from the process memory object, which
is currently the slowest way to read memory. Change the code to read from
the elf memory object which will almost always be a memory mapped file and
should be really fast.

Also, move the tests for StepIfSignalHandler to its own file.

Bug: 23762183

Test: Unit tests pass.
Change-Id: I1100b50554e5ef736a87babd484f3f34e9071caa