Add tests for in-transition back navigation retries

Adds unit tests for the `TransitionIdleRunner` logic, which handles retrying a call to `startBackNavigation` when the system is already in the middle of a transition.

The new tests verify:
- A retry is correctly scheduled and executed when transitions become idle.
- A scheduled retry is correctly cancelled if the back gesture finishes before the retry can be executed.

Please help fill out the survey for feedback: https://docs.google.com/forms/d/e/1FAIpQLSeKFKpHImCAqZIa_OR801cw72HQUreM2oGM25C3mKKT2tBFnw/viewform?usp=pp_url&entry.1586624956=ag/35413236

Please feel free to use your domain knowledge to make changes to the generated tests to make it more valuable for your team.
Original Change: ag/34901827
Test: ATP tests passed http://go/forrest-run/L23200030017392531
Bug: 431235865
Flag: TEST_ONLY

Change-Id: I4654e41f2c8fe18347201dc469d183d428240eda