Add padding to height of AutoCompleteTextView drop-down.

As described in http://b/issue?id=2017825
PopupWindow.getMaxAvailableHeight() does not include the padding in
the available height. To get the ACTV drop-down to actually fill
the screen, this change adds the padding to the value returned
by PopupWindow.getMaxAvailableHeight().

This is part of the fix for http://b/issue?id=2014450