MetaKeyKeyListener: Disable locking of toggled modifier keys

Toggled (or 'sticky') modifier keys that lock when pressed twice
cause unexpected behavior that confuses users. Shift is especially
problematic, as there are three distinct Caps Lock states in
Android: The Caps Lock key, Alt + Meta, and Shift pressed twice.
Only the first illuminates the Caps Lock LED, and each can only be
disabled by pressing the right key/combination again. This forces
users to 'hunt' to find the right one, and pressing Alt twice will
lock that modifier leading to yet more unexpected behavior...

Get rid of locking for toggled modifier keys to improve this mess.

Change-Id: I94de5d8d051b2d518457a1a44c9ee939508b9abb