The back key stops selection mode in extracted text mode.

The previous behavior stopped the extracted text mode, leaving the
text selected and without handles in the app.

As what happens in normal (non extracted) mode, the back key now
stops the text selection mode. A second back will get the user back
to normal mode.

Change-Id: I2e8d2d7a1a1e1344997da75438f8df804fb8735c