[dev_option] Move DestopModeFlags out of android.windows.flags

This is to maintain separation of non-generated files from generated flag values.

Make sure cache is resetted after test run in DesktopModeFlagsTests so
as to not interfere with other test files.

Test: Current tests pass
Flag: EXEMPT refactoring
Bug: 370777204

Change-Id: I2eff92db445afce22ad06d2631d94e775ae2e055