Recents : Fix clear all layout

Fixes the clear button from getting cut off due to the added
fitsSystemWindows in aosp
Allow volumes to be unlinked. Make the volume overlay more functional
GlobalActions: Fix possible race condition when showing dialog.

Depending on the presence of a previous dialog, the newly generated one
gets either immediately displayed (no old dialog) or shortly delayed
(after dismissal of the existing one).
If a new show event happens (by rapid fire of the power key, for instance,
using a keyboard with fast-repeating events) during that delay, a new
dialog will be immediately displayed, and when the delayed show triggers
it'll conflict with it (and bring everything down).

This moves the creation of the new Dialog object to _before_ showing
it (delayed or not), instead of the show method itself, to ensure the
object isn't clobbered by competing/sequential calls to handleShow()
Enable multi-user support for volume_link_notification
shorter glow animation