Defer AudioTrack initialization to first ref

Part of AudioTrack initialization involves sharing sp<> based
refs to itself. If this initialization occurs during construction,
races leading to pre-mature destruction and use after free errors
are possible since sp<> creation requires a fully created object.
Push this initialization (calling set) to onFirstRef.

Test: atest AudioTrackTest
Bug: 221393500
Change-Id: Ie28162ec4a975206cb81d13ec3afa930fbd1675c