Add base class for desktop e2e test scenarios

- Add a base test class for Desktop test scenarios
    - Add ScreenRecordRule to that base test class.
    - Make all desktop test scenarios extend that base class
    - only keep screenrecordings on test failure
- Change the test config for WMShellFlickerTestsDesktopMode tests to
  pull screenrecord files (created from the test-rule above) to the host
  after each test.

Flag: EXEMPT test update
Bug: 405886707
Test: atest WMShellFlickerTestsDesktopMode

Change-Id: Iec3df9252e82b6e02ba76401b96d6634f801fc22