Add accessibility titles to PopupWindow

Adding a default title and a hidden API to customize
the title.

Bug: 30177642
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: I38591a25be0fe1d5bf57c9edee29f09c92e258ea