Fix S MTS on Q by excluding unavailable hidden API.

Reason for test failure: ag/12839348 updated call to DragEvent.obtain
API which is a hidden API whose signature has changes in S. We should
exclude the test cases for S MTS in pre-S.

Bug: 182963154
Test: atest DocumentsUIGoogleTests
Change-Id: I356322a7725c91a9668cdf8626aa87e97c50b992
(cherry picked from commit a121511b)