Make the search dialog's autocomplete dropdown behave correctly

with respect to the soft keyboard. getMaxAvailableHeight in
PopupWindow relies on View's getWindowVisibleDisplayFrame, which
always takes into account the IME, whereas in this case, because
we want the window to extend down to the bottom of the display
when it is being interacted with by the user, I added a new
boolean option 'ignoreBottomDecorations' to getMaxAvailableHeight.
If true, the method returns the height which will extend the
PopupWindow down to the bottom of the screen, rather than just
down to the IME.

Then in AutoCompleteTextView, I set this to true when the
dropDownAlwaysVisible attribute is true and the PopupWindow's
input method mode is INPUT_METHOD_NOT_NEEDED (which happens when
the list is dragged or otherwise interacted with).

Then, because the dropdown height is only calculated when we call
showDropDown(), I had to replace a couple calls to mPopup.update().

There are still a few remaining bugs here I'm tracking Still to do:
  * Fix a strange re-placement of the window which happens
    intermittently.
  * Strange behavior when using the directional pad.
  * In a few cases, the list does not correctly size itself to the IME.
    This seems to be because the available height is calculated before
    the IME is yet on screen, and thus is calculated as the entire screen.