Remove TrackBase::mFlags

The bit-field TrackBase::mFlags was supposed to have track-specific
flags in the upper 16 bits, and system flags in the lower 16 bits.

The upper 16 bits of mFlags were initialized in the TrackBase
constructor from the flags parameter of IAudioFlinger::createTrack()
and IAudioFlinger::openRecord(), and the lower 16 bits were cleared.

However, the upper 16 bits of mFlags were never acccessed again.
So really there are no track-specific flags.  I left the flags
in the parameter list of createTrack() and openRecord() but made a
note that these should be removed eventually as they are dead.

This leaves only the one system flag "step server failed".  I replaced
the bit-field mFlags by bool mStepServerFailed, which is simpler and
slightly faster.

Change-Id: I6650f5487be72791b4a67d73adcd10ffa04e2aa5