Add animation for power saver dialog

This dialog appears the first time Power Saver is enabled, if it's
triggered from QS.

Because there's a roundtrip through SettingsLib and a
broadcast, we cannot just pass the view. Instead, save a (weak) reference to
the view in the controller that can be retrieved when it's time to show
the dialog.

Test: manual
Test: atest SystemUITests
Fixes: 225183234
Change-Id: Ife049c5ce4eaf7823cd8ba636f511ada401308e3