Implement the Photopicker Transcoding logic in Photopicker Application

Implement video transcoding feature in photopicker which helps Apps to
handle incompatible format. Apps can trigger this feature by providing its
ApplicationMediaCapabilities in intent extra.

To make the media preparation (remote file preload and video
pre-transcoding) flow and dialog display correctly, the media preload
related logic is separated from the CloudMediaFeature and extended
into a new feature MediaPrepareFeature. Also, to make it more general,
"preload" is replaced by "prepare" in the naming of some variables,
methods, and classes.

For details see: go/photopicker-hdr-video-transcoding.

Test: manual test & TranscodeHelperTest
Bug: 367583370
Flag: com.android.providers.media.flags.enable_photopicker_transcoding
Change-Id: I784634af656ebad072bee8314d6ea20f1df07229