Add mocking support for now function in AttributionProcessor

In the regression test in changeid: I1709af943b6fa238dd4df41a62e6add36984c9ec
The triggering of the code we want to test depends on  the return
value of std::chrono::system_clock::now(). To facilicate testing,
in this patch we add a now_func_ (a std::function) field in
AttributionProcessor and make it call it instead of
std::chrono::system_clock::now(). Mocking `now` is made possible
by passing a custom function to the constructor of AttributionProcessor.

Bug: 254774758
Test: refactoring, existing tests still pass
Ignore-AOSP-First: security
Change-Id: I7dd3a0e665f72c27e4d1844f45ec15a8dd1ddb53