C2SoftOpusEnc: fix misplaced output timestamp computation

During a process call, at the start of the loop, the current frame pts
is computed. At the end of the loop, if there are no more samples to be
processed, then the work item is filled and returned. If there are more
samples to be processed, this timestamp is marked as previous output pts
and used during cloneAndSend() and the current output pts is updated.
The problem occurs, if there are more samples to be processed but not
enough to generate an output. In this case output pts need not be
updated. The previously generated timestamp should be used to signal the
output and mark the work item as done. This is addressed by deferring
the output pts computation only when an output is generated.

Minor code cleanup.

Bug: 412479079
Bug: 139484367
Test: atest MctsMediaV2TestCases -- --module-arg \
 MctsMediaV2TestCases:instrumentation-arg:media-type-sel:=opus
Test: atest MctsMediaEncoderTestCases -- --module-arg \
 MctsMediaEncoderTestCases:instrumentation-arg:media-type-prefix:=audio/opus
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest
Flag: EXEMPT bugfix

Change-Id: I42162b42587927aed727da6a9eb790835105d221