media: allow excluding arbitrary codecs from REGULAR_CODECS list.

This can be done by adding the 'special-codec' feature as a required
feature to media_codecs.xml, e.g.

<MediaCodec name="..." type="...">
  ...
  <Feature name"special-codec" required="true" />
</MediaCodec>

This feature is not exposed to applications, and is only used to
exclude a codec from the REGULAR_CODECS list.

Bug: 197577115
Bug: 196518411
Bug: 191944087
Test: CtsVideoTestCases and manual testing with a modified media_codecs.xml
Change-Id: Ica5510bbe7e781f25ca329535d2ba771afbdb263