Update battery saver warning

This warning needs to be shown if a user tries to set a Battery
Saver schedule without having seen it so we make the show dialog
method public. Addtionally we make it possible to trigger a more
generic version of the notification that blocks whatever action
the user was trying to take and only marks if they confirmed
the notification or not. Once the user has acknowledged the
notification they can repeat their action to get the desired
result. Text is updated in the generic version to make it
more clear that the dialog is just information and has no
effect.

Test: robotests pass, manual verification
Bug: 128924009
Change-Id: Ia4e7935de029f8e74e5236d8bf1651a53c5021c6