AudioFlinger: fix stop detection for static tracks

The end of playback and end of presentation detection was broken for
static AudioTracks (tracks using shared memory buffers passed by client).

The mixer should not wait for a minimal amount of frames to be available to mix
a static track otherwise the last frames might never be consumed.

A static track should be removed from active list in case of underrun even if not
stopped().

Issue 6411521.

Change-Id: I66a2c1a77e98149e5049a223a6f04c3b8c5ad11a