Set PageIndicator pages in QSPanel#onMeasure

Pre-emptively update PageIndicator's numPages before measuring in
QSPanel so that views will be appropriately measured. Previously, new
PageIndicator child views would be added in the middle of a QSPanel layout pass
after already being measured, so the new PageIndicator child wouldn't be
measured.

Test: manually add and remove new QS tiles. rotate between landscape +
portrait modes. observe the PageIndicator is correct.
Fixes: 157542221

Change-Id: Ic1bb29039dd12993a50758bf48a4ad792d8cdfc3