Clean up popup window titles

Removing the recently-added APIs to adjust the title for
the popup window, and moving setting the title to
preparePopup(), which is guaranteed to have a context.
(The bug is a crash because we didn't have a context.)

Apps wishing to specify their own title can still do so
in the LayoutParams without bothering with the get/set.

Bug: 62675086
Test: With TalkBack running, navigate to Settings->
Network & Internet. Select the more options button and
double-tap to select it. It now announces "Popup Window".

Change-Id: I9735ace42c24ef4977d3f4d5150f768d44aff351