introduce binderRpcBenchmark

VRRRRRRRRRRRRRRRRRMMMMMMM!!!

Some caveats:
- we are partially benchmarking the scheduler
- this is a microbenchmark

Results running on host (we'll want to watch and tune these on real
devices with core clamping and thermal throttling off):

-------------------------------------------------------------
Benchmark                   Time             CPU   Iterations
-------------------------------------------------------------
BM_getRootObject        45959 ns        39054 ns        18345
BM_pingTransaction      26128 ns        23360 ns        28705
BM_repeatString        394286 ns       200186 ns         3508
BM_repeatBinder         39357 ns        37437 ns        17705

Bug: 182940634
Test: run benchmark on host and on cuttlefish
Change-Id: I5084a00ddf207eaeb803da98587ca04df65add4d