Include AudioPolicyInterface.h only when needed.

Both legacy and non-legacy AudioPolicyInterface define
extern "C" functions createAudioPloicyManager and
destroyAudioPloicyManager. Standard C++ does not
allow overloading of extern C functions.
g++ did not catch this problem but clang++ does.

Change-Id: I80869cdb26e7721777e53d4eff71c5c733044abf