Initial port to cobalt 26

This also includes some source changes required due to functionality
no longer available on cobalt 26:
- removed specific check for vp9 support
- removed injection of some accessibility events
- renamed some exported symbols to use new naming scheme
- replaced use of starboard::Mutex/ScopedMutex/ConditionVariable

Apart from general updates to use new namespaces, drop includes no
longer available, etc.

b/435468447

Change-Id: Ie4ec6e9f528db90b1b21aca2fb2dea2b92b71fd2