Camera2: Ignore exception on double-completed capture sequences.

As a workaround for b/27870771, convert fatal exception into a
warning.

Duplicate calls to finishPendingSequence with the same sequenceId
are seen in user crash reports, but with no further context and
inability to reproduce after testing various hypothesis, just
warn about it happening.

Bug: 27870771
Change-Id: Icd7d408887e04b94092689ce61809d6c664d8e3a