On desktop, open files in a new window

The desktop behaviour relies on `documentLaunchMode` which is implicitly
enabled in all apps unless they override it in their manifest. Apps that
set `documentLaunchMode="never"` will still open in a new window but
will have a weird behaviour if two documents are opened using the same
app (i.e. opening the second document will bring the app window to the
front but the app will still display the first document). See
https://issuetracker.google.com/issues/373759691#comment3

Bug: 373759691
Flag: com.android.documentsui.flags.desktop_file_handling
Test: Unit tests and manual testing

Change-Id: I9d21364879e4f3de780b207365da9bac97ff47ce