Adding app widget restore support in Launcher3 for android L

When the app is restored, it displays placeholders for all pending widgets.
These placeholders can be moved and removed similar to a widget (size is fixed
to what defined in backup). Once the system notifies the launcher of the new
widget ids, the place holders are replaced with actual widgets.

issue: 10779035

Change-Id: I68cbe4da01e9ca2978cb4471a7c645d2aa592055