Add "Open" to context menu when a single file is selected

The open context has a slightly different behaviour to normal click.
Normal click does a ACTION_PREVIEW, fallback=ACTION_VIEW but the
context menu does a ACTION_VIEW without a fallback.

Reusing an existing menu item which is unused. This was disabled for
* picker in http://ag/4269998
* files in http://ag/4056064

Bug: 384607914
Flag: com.android.documentsui.flags.desktop_file_handling
Test: Unit tests and manual testing

Change-Id: I60b7a6a1cfaf0fe5f2b534341296a7c6322f4371