DocumentsUI: Allow open files instead of URIs

Check files' real path and mime type to open them only in standalone mode.
This allows, for example, installing apks.

Change-Id: I4501c694db84b8595cd3928752254a8914539972
Merge "DocumentsUI: Allow open files instead of URIs" into android-4.4
Merge remote-tracking branch 'omni/android-4.4' into android-4.4