Clean up error handling in createTrack and openRecord

Outside callers now use initCheck() to determine whether a TrackBase
has been created successfully, instead of relying on internal knowledge.

Previously, callers needed to know that a TrackBase was only valid if it's
getCbk() != 0.  For a Track (playback), they needed to know to also check
the track's name (track index).  Now, outsiders can just call initCheck().

Other changes:
 - Return a 0 reference if track creation fails
 - Remove a dead line of code in AudioFlinger::openRecord

Change-Id: If374924a3f6fd27906f625aa83dd0a1e3f506e00