Take in test instance as field.

This refactor makes it simpler for the rule to keep track of the test
instance (immutable). Having the rule be a TestRule instead of a
MethodRule also simplifies nesting TestRules within this implementation
(future CLs).

Test: Taskbar{AllApps,Overlay}ControllerTest
Flag: EXEMPT refactor
Bug: 230027385
Change-Id: Icf587202242fcb4526946d9c7619e5be4f59154e