Notify DecorContentParent about content change

ActionBarOverlayLayout which implements DecorContentParent would stop
dispatching the WindowInsets dispatched from its parent view, and then
will dispatch it during onMeasure when there is any insets change.

When the content view is changed, the ViewRootImpl will re-send the
WindowInsets to the hierarchy. However, ActionBarOverlayLayout would
stop dispatching it because the insets doesn't change. So that the new
content view won't have a chance to apply the insets.

This CL notifies ActionBarOverlayLayout when the content view is changed
and reset mLast[...]Insets used to compare the insets change.

Bug: 328745925
Flag: NA
Test: atest MagnifierTest
Change-Id: Iaf90447284f897fa2607984c36e12d596f7976c5