Allow WindowManager.TYPE_SCREENSHOT to be focusable

Pre-R, the screenshot window was used just for screenshot animation. In
R, the window is also hosting tappable screenshot actions, which require
focus.

This change modifies TYPE_SCREENSHOT to no longer force it to be
unfocusable.

Test: Verify that screenshot window UI elements can work with talkback
    and accessibility scanner.
Bug: 153517161
Bug: 152583784
Change-Id: If81d9f94dff801c3483a2d834e692b4c77d80d7b