aaudio_loopback: port latency analyzer from OboeTester

Integrate with aaudio_loopback.
Eventually this could be used in the CTS Verifier for latency testing.

Pass std:string back from analyze().
General cleanup.

Test: adb shell aaudio_loopback
Test: check the latency.msec value
Change-Id: Ibf7128d78d47d91e0bf314344ca450d7f70b3ceb