Show ROOT directory on Scoped Directory Access Sample.

Test: ./gradlew preflight && ./gradlew assembleDebug && adb install -r \
      ./Application/build/outputs/apk/debug/Application-debug.apk && adb shell \
      am start com.example.android.scopeddirectoryaccess/.MainActivity

Bug: 63720392

Change-Id: I7aa14b4e71fa5ce426848c138ac404e06310ba3c
(cherry picked from commit 1e701e91)