aaudio: convert I16 input to float

This allows an app that requests a LOW_LATENCY FLOAT INPUT stream
to get a FAST track when using Legacy mode.
This is needed because Legacy AudioRecord does not allow FAST for FLOAT
streams.

Bug: 38268031
Test: adb shell input_monitor -m1 -pl -f2
Test: adb shell input_monitor_callback -m1 -pl -f2
Test: On Pixel phones you should get actual performanceMode=12
Change-Id: Ia5879ff4904f50bbb5009a3fc058800e53770710