MediaBrowser2: Fix naming inconsistencies

This CL modifies following APIs:

1. getLibraryRoot() related
 - onGetRoot => onGetLibraryRoot
 - onGetRootResult => onGetLibraryRootDone

2. getItem() related
 - onLoadItem => onGetItem
   - Also renamed argument: itemId => mediaId
 - onItemLoaded => onGetItemDone

3. getChildren() related
 - notifyChildrenChanged/onChildrenChanged
   - Renamed argument: childCount => itemCount
 - onLoadChildren => onGetChildren
 - onChildrenLoaded => onGetChildrenDone

4. getSearchResult() related
 - onLoadSearchResult => onGetSearchResult
 - onSearchResultLoaded => onGetSearchResultDone

Bug: 73711101
Test: Passed MediaBrowser2Test
Change-Id: I7f59686e21f4de9d4512cd19067c944e49faad39