Use @TestApi of ActivityManager instead of am command

Although there is no remarkable speed gain.

Test: All tests pass.
  $ cts-tradefed run singleCommand cts-dev \
      --module CtsWindowManagerDeviceTestCases \
      --test android.server.wm.CrossAppDragAndDropTests
Bug: 70190274

Change-Id: Iea27e55c89bbb90708b388bd08eeb164d0865e3d
1 file changed