MediaSession2: Add onDataSourceMissingHelper

This allows a developer to create DataSourceDesc when the item is about
to be played. Typical example of the usages are,

    1. For a playlist consists of FileDescriptors, its developer may not
        want to open all files when MediaSession2.setPlaylist() is
        called.
    2. A controller has called setPlaylist(), addPlaylistItem(), or
        replacePlaylistItem(). Controller cannot know the
        DataSourceDesc, and only the session developer can know about
        it.

Bug: 64098437
Test: Run MediaComponents test
Change-Id: I73f27ca0a799b1cddf5046b41f0ca01d08037103