Handle some edge cases when seeking while starting up OMXCodec

These were exposed by the new preview-seekframe while paused code.
In particular, the codec may have been in state RECONFIGURING when attempting
to seek, or we may have initiated flushing of the output port and this may not
have completed yet by the time we want to reconfigure the output port.

Change-Id: Id7640ade11dbc7205a22f648ea0b5e3e9b49cf4b
related-to-bug: 3392259