A2DP: Don't start or suspend the encoder for a non-active sink

While switching the A2DP active device, those AV_START or AV_SUSPEND
events might be from previous active device which was no longer active
now. We should keep it as suspended, but don't change the encoder and
audio provider state by these events.

This CL also simplify A2DP source suspend / stop callbacks.

Bug: 144670756
Test: Switch A2DP active device manually
Change-Id: I640e1e219306641beca1ff5ac758c2c95d925737