MediaBrowser2: Add search related methods

This CL adds following APIs:
 - BrowserCallback#onSearchResultChanged
 - BrowserCallback#onSearchResultLoaded (previously onSearchResult)
 - MediaLibrarySessionCallback#onLoadSearchResult

This CL also fixes conflicting @NonNull/@Nullable annotations in
MediaItem2 methods.

Bug: 72786724
Test: Builds successfully
Change-Id: Ica90ca6b47f33cd3d457a28598a035a00d144dfa