Add documents UI CTS test to MTS.

Test: m mts, mts-tradefed run mts-documentsUI
Bug: 139495844
Bug: 123411735
Exempt-From-Owner-Approval: Already approved in
https://googleplex-android-review.git.corp.google.com/c/platform/cts/+/9208364

Change-Id: Iccc69ffc0463e5447da1c7362e7fab45f09dab4a
Merged-In: I28c59ee7d7e55324b9a3749b3fe000c53f6876cf
4 files changed