[A11y] Magnification key handler checks if mag is active

Magnification key handler is installed when magnification is enabled,
so it steals key events whether or not magnification is active as
long as the correct keys were pressed. Adds a method to the callback
to check for magnification activated, so that key events are only
consumed when magnification is both enabled and activated.

Also adds a check that ctrl isn't held down with action and alt,
so that ctrl+action+alt+arrows doesn't get consumed.

Bug: b/410887351
Flag: EXEMPT bugfix
Test: atest MagnificationKeyHandlerTest and manual with TalkBack
Change-Id: Ief65b8b70138625148ee5254c318113174c6f0a5