[DocsUI Peek] Initial Peek overlay and view manager

Define the initial Peek fragment, with its associated layout.

If Peek is enabled, the PeekViewManager, instantiated when DocsUI is
launched, loads the PeekFragment (with its inflated view) in its
hidden container.

Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.use_peek_preview_ro
Test: DocumentsUIGoogleTests:com.android.documentsui.peek.PeekUiTest
Test: DocumentsUIGoogleTests:c.a.d.files.ActionHandlerTest#testShowPeek
Bug: 382163291
Bug: 382163274
Change-Id: I7fdd1afd856803091425b8257b69206746530989