nuplayer: support widevine sources

- handle widevine:// scheme
- add separate looper for renderer (as it can block initial buffer
  handling if all buffers are used)
- initiate secure codecs before source is started
- don't read secure buffers
- share ACodec's input buffers with Widevine source

on the decoder side

- keep track of mediabuffers released by widevine source
- keep track of dequeued input buffers (for safety)
- release mediabuffer when buffer is subsequently dequeued.  (This
  was hardcoded into OMXCodec to do this when buffer-empties message
  was handled, but MediaCodec does not support such functionality.)

Bug: 15699665
Change-Id: I4a369443294e45c644be8b0257010e52db1d7c9b