AudioManager: make AudioPortEventHandler static

Make AudioPortEventHandler and audio port and patch caches
static members of AudioManager. There is only one callback
per process in AudioSystem for audio port updates
and having those non static would not work when more than
one AudioManager instance exists in one app.

Bug: 18727023.
Change-Id: I4c1041dc6441d168be4efa066e14289cc5f41872