Add DocumentsUI unit tests to MTS

Change targetprep to SuiteApkInstaller so that the MTS suite can find the apk.

Changing the targetpreparer will not affect previous apk searching for device-tests.

Test: m mts, mts-tradefed run mts-documentsUI
Bug: 139495844
Bug: 123411735
Change-Id: Ife1f9dfcdb2265dc1e07a937b90ffe6140563171