Change WAV file folder to a subfolder of Context.getDataDir().

The old mechanism of Environment.getExternalStorageDirectory() doesn't actually
allow files to be created in that location.

Bug: 349620497
Test: Run Audio Loopback Latency Test. Look in folder displayed in Results panel for saved WAV files.
Flag: TEST_ONLY
Change-Id: Ic6de4f5bd9bc63895b35a79919a4859ff0d78bf0