On BACK in search, hide just IME if IME is fullscreen

Since a fullscreen IME covers the entire search dialog, there was
no discoverable way to get back to the search dialog.
Now, on BACK presses when the IME is fullscreen (e.g. in landscape
mode without hard keyboard), we dismiss only the IME.
A second BACK press will dismiss the search dialog.

Fixes http://b/issue?id=2159132

Change-Id: I8e5736d83d7c35936a718929017ea04c6214700b