stagefright: rework metadata buffer support

- Add fence to new metadata buffer mode structure, but don't use it
yet, so that we don't have to add another mode soon.
- Change GraphicBuffers to ANativeWindowBuffer in new metadata mode.
This removes internal depencency from vendor codecs.
- Extend new metadata mode from SW encoders to all codecs.
- Fallback from new metadata mode to old mode (so Camera can always
use the new mode even with legacy codecs)

Related-bug: 19614055
Bug: 13222807
Change-Id: I405a21dd6ce8e99808f633bfeab8f14278eb7079