Use a weak reference to the context in InputManager

It has been reported that a recent change to InputManager is
causing a memory leak. We were already planning to change it in a
way that this issue would no longer happen, but since it's a
refactor that will take some time, this is a temporary solution
that Prabir suggested; if we keep a weak reference to the context,
it will be freed when there is no other active references.

Bug: 267241786
Test: will ask for a test run of this change
Change-Id: I7e894ba1f014bba36f6ea2566b11426d17d84f68