Remove all desks before starting test.

The testSetupRule runs a command that clears all recent tasks but
doesn't remove them from the repository. Run removeAllDesks to ensure
the test starts from a clean state.

Fixes: 424961460
Test: atest
Flag: EXEMPT test fix
Change-Id: Iae89c477e0566b39ba1e20b27c993e594c3f6b86