aaudio: use fmin and fmax to block Nan

This prevents NaN from creating noise that does not respond to
Volume changes.

Bug: 69224661
Test: adb shell aaudio_loopback -c2 -C2 -tm -m2 -s20 -Pl -pl
Change-Id: I8604f5af42cbf5637f7ea4e975f53db990cdf9de
1 file changed