Migrate packages/apps/DocumentsUI to androidx.test

See go/jetpack-test-android-migration

Test: make checkbuild
Change-Id: Iee68768060f3811678eb51318fb429b629ff03b1
diff --git a/tests/README b/tests/README
index 7112376..d8ea236 100644
--- a/tests/README
+++ b/tests/README
@@ -8,6 +8,6 @@
 and uses it instead.
 
 To run just small tests"
-adb shell am instrument -w -e debug false -e log false -e timeout_msec 300000 -e size small com.android.documentsui.tests/android.support.test.runner.AndroidJUnitRunner
+adb shell am instrument -w -e debug false -e log false -e timeout_msec 300000 -e size small com.android.documentsui.tests/androidx.test.runner.AndroidJUnitRunner
 
 Replace '-e size small' with '-e size large'.