Add flag for keyboard backlight shortcuts

Bug: 401626732
Flag: com.android.hardware.input.keyboard_backlight_shortcuts
Test: Add code that uses the flag and verify that it is properly
guarded by the flag. Change the flag on a DUT and verify that
the code is executed.

Change-Id: Ib6e064192e1ab2885c8bb02f56c2a07e4bf09f87