audioflinger: fix static track end detection

If a static track is not a fast track,
prepareTracks_l() must rely on framesReady() to
detect end of buffer and remove the track from the active
track list.
Failing to do so results in the track staying active but
not processed by the mixer because in underrun. This leaves the
mix buffer content uninitialized and causes the effect process
function to accumulate its output onto undefined data.

Bug: 12013676.
Change-Id: Iad72c921fa18d34811abf7d1073890c093a27725