Allow injecting bubble controllers in taskbar test

Update TaskbarUnitTestRule @InjectController annotation to support
injecting bubble controllers.
Bubble controllers are optional and only initialized if the feature flag
is enabled.
Add a test to check that bubble controllers are only initialized when
the flag is on.

Bug: 371017282
Test: TaskbarUnitTestRuleTest
Flag: TEST_ONLY
Change-Id: Ibea2dbe0db854d530ef93ac53204a361572fa5b2