PopupWindow: Don't ignore top decorations in getMaxAvailableHeight.

getMaxAvailableHeight is ignoring the top insets, while
findDropDownPosition is not. This is causing getMaxAvailableHeight
to return a fits above position that findDropDownPosition will think
is too large.

Bug: 31048766
Change-Id: Ifa57cb4ebe0944c701a6f38b58d4f144d8b9199c