Add new benchmark options.

Add a bunch of extra options to allow greater flexibility for creating
benchmarks. Add a pattern option that can be used to represent any
one buffer or two buffer possibility. This should fully replace the
functionality of microbench.

Add a single option for one buffer benchmarks and two buffer benchmarks
that represents all of the possible options to run a string benchmark.

Add a string test suite that includes all string benchmarks using the
above option.

Test: New unit tests to cover all the new options, and all pass.
Test: Ran new string test suite with a single bionic iteration.
Change-Id: Idb13ea15e44cec626e9f46672ccd648d7ca72ba6