audioflinger: Allow bundling the A2DP interface

The A2DP interface is usually built into the audiointerface static
lib, which gets integrated into libaudio.
However, if we're using a proprietary binary libaudio, we may not
want to use the A2DP implementation that comes with it; this
patch (enabled by the BOARD_FORCE_STATIC_A2DP option) forces
libaudioflinger to include its own A2DP implementation and ignore
the proprietary one.

Change-Id: Ib807109f668854bcae1dbf3f1e870c969f6c30c0