libstagefright: Set is_streaming flag for all widevine content

- For widevine clips played from sdcard get the file type
  from extractor's meta data.
- By setting is_streaming flag 2k buffer size will be used
  for audio offload.

CRs-Fixed: 615323

Change-Id: Ic54a642aa70c3355583807eb4ca2034e4207dfd4