Audio focus: replace durationHint by focusReqType

Replace the durationHint parameter, the old name for the focus
request type, by focusReqType that better matches the
AudioFocusRequest API and parameter description

Bug: 324780808
Flag: EXEMPT cleanup
Test: m
Change-Id: I1f53d8b66c7dbfcfe01eca56f2436b09b3025e14