Add missing return in Tuner default impl getAvSharedHandle after hidl_cb

Per CTS automation test failure logs:
Abort message: 'getAvSharedHandle: _hidl_cb called a second time, but must be called once.'
this should be the root cause of the failure.
Couldn't reproduce locally.

Test: make
Bug: 181997400
Change-Id: Ib3fb5ad535ab51cc7281c3f81a61d1913a1f4e65