aaudio: fix detection of input overflows

Was incorrectly looking at the buffer size instead of the buffer
capacity.

Test: OboeTester ECHO INPUT to OUTPUT
Test: set Input margin to 4
Test: XRun# for input should stay at zero
Bug: 141700208
Change-Id: I21ecdac19f5361a1b33f4dc2aaec170de9e28fc6
1 file changed