Fix the seesaw behavior on the status panels.

The code in onMeasure that attempts to accommodate changing
child boundaries (i.e. an expanding/collapsing notification)
was locking the window size to the child measurement, even
though another window was trying to seesaw it closed. We now
only activate the rubberband behavior when the size of the
children changes, not on each onMeasure().

Lots of other small fixes to the panel expansion logic, too,
including ensuring that the expanded fraction doesn't jump
to 1.0f when the panel content hasn't been laid out yet
(causing the entire screen to become dim for a second thanks
to PhoneStatusBarView.panelExpansionChanged).

This change also restores the ability of Binder clients
(such as accessibility systems) to expand and collapse the
status bar.

Bug: 7008196
Bug: 6995518
Bug: 6628429
Change-Id: Ib0244a27aef2b4dbe19be98f7e039a9d38f1b80e