DashCodec: keep cache of formats to support b2b format change

     - when format change takes place too fast, it will pick up wrong
       format.
     - keep a cache of formats and only issue one sendFormatChange per
       crop change.
     - clear format cache when flushing

Change-Id: Ib45f3c5ac7764b4dd1edc4ac6c6f11390c33c151
CRs-Fixed: 475092
Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>