Allow current IME to start background activity

Current active IME should be always allowed to start activity,
like IME settings.

Bug: 159433730
Bug: 168397303
Test: Click home button, then click on any edit text on home screen and
open IME setting from IME, setting app is not blocked / delayed.

Change-Id: I959cdca58489701453f645a02de0c4879661029b