aaudio_loopback: do not set buffer capacity by default

That was causing us to not get a FAST track.

Bug: 112186931
Test: adb shell aaudio_loopback -te -pl -Pl -m1 -n2
Test: should get PerformanceMode 12 on INPUT
Change-Id: Ib8b00585a80bdd86f752a5ee37aacd92f06db347
Merged-In: Ib8b00585a80bdd86f752a5ee37aacd92f06db347