Don't extend alert windows onto status bar/nav bar.

This requires preserving information about the status bar/nav bar insets
and using it to reduce the available space when measuring wrap_content
windows.

Also remove the hack that was introduced for round devices and should be
handled instead by the round qualifier and theming.

Bug: 26014496

Change-Id: I5688c50153047dcabc197adab8fec141630a2b51