Don't update prepare state more than once

Don't update prepare state more than once

If a MEDIA_ERROR event is received during preparation, it indicates
that prepare has completed unsuccessfully.  However, MEDIA_ERROR can
also legitimately be sent after successful prepare. In that case, don't
update the prepare state.

Similarly, in the unlikely case that MEDIA_PREPARED is received after
prepare has already completed successfully or unsuccessfully, ignore it.
This probably indicates a buggy mediaserver, but we wouldn't want that
to take down the client also.

Change-Id: I2a92e98d13b70d1aa5ff6f440ad528f6896a2ead