Fix bug 2927837 - PopupWindows don't account for xlarge status bar height

Add framework dimension resource screen_margin_bottom. This describes
the bottom margin formed by permanent screen decorations. In xlarge
configs this is equal to the height of the permanent system bar.

PopupWindows now size and position themselves with respect to
screen_margin_bottom.

Change-Id: I8960059adcf09605abf26c228bf877f5632fd11e