AssistContentRequester-weaken references to system callback

The callback objects used to call into system server can
outlive the parent requester object. This can cause leaked
activities (on rotation for example). Use a weaker collection
of callback objects that aren't tied to the lifecycle of
the callback.

Bug: 186670923
Test: Ran TAPL tests to check for leaked activities
Change-Id: I33f556ad55b823652b06410d264f1473d3003320