Split scenario and tests on flicker service tests

This is necessary to simplify running, identifying and maintaining failures, as well as improving module isolation.

Key issues:
- Split screen platinum, benchmark and flicker tests were clustered together in a confusing way
- Desktop mode and split screen tests were in the same module and flakiness in one type of tests affected the other

The new format is:

```
shell/tests/e2e
--------> desktopmode
----------------> scenario definition
----------------> flicker-service
--------> splitscreen
----------------> scenario definition
----------------> flicker-service
----------------> flicker-legacy
----------------> platinum
--------> <others still to be moved>
```

Flag: TEST_ONLY
Test: atest WMShellFlickerServiceTests
Change-Id: If33da2953ac4f6829c973c7414c4e6c88a1c7f69