audioflinger: only update mHalVolFloat on success

This will fix a regression with AAudio MMAP stream not getting
its initial volume set correctly.
Also initialize mHalVolFloat.

Bug: 74547253
Test: adb shell write_sine_callback -pl -s10 -m1
Test: should have the same initial volume as
Test: adb shell write_sine_callback -pl -s10 -m3
Change-Id: Ie0c1f9ba51e2059a888d2273f41588f101b09a5c