Password bouncer - Go back on first gesture

When using the bouncer with password security, it currently takes 2
swipes to go back: 1 to dimiss the IME, the 2nd to exit the
bouncer. For this case, intercept the back key event and immediately
close bouncer.

Fixes: 174020768
Test: manual
Change-Id: I85a650bb4a28d4209dc84461e69fb80824228a39