Fix 3374446: Allow multiple shift keys in Keyboard.

This fixes a bug introduced in 3c6dd8f9 because we now
have two shift keys. The code now tracks a global state
and looks for up to two shift keys.

Update after review and added code to handle extra
invalidate required by additional shift key.

Change-Id: Ic1728dd0ceec089089cd1beca1b0b30565d6e658