DocumentsUI: Enable more tests for Desktop post-submit

The primary-device keyword is a tradefed filter that runs only on flame
devices, which isn't desktop, so removing the keyword allows to run the
target that has more than just unit tests.

Flag: TEST_ONLY
Test: atest DocumentsUIGoogleTests
Bug: 383617339
Change-Id: Ie2f27e7446f22e28c93130ece2b90d6d5a70b56b