Debugging for issue #2250075: Desk dock clock app sometimes doesn't

This adds a history of the last 100 broadcasts that is printed in the
debug log, to be able to see what recently happened at the time the
bug report was taken.  Also does some optimization of the printing of
the broadcast records to make it feasible to print this number of
entries.  (We kind-of need to do this because there are some broadcasts
like SIG_STR and SYNC_STATE_CHANGED that are being broadcast a LOT.)

Change-Id: I775e1ec0f63369c8bca8c83cee27b95ddc5ec450