Hide the a11y service enable dialog from non-a11y-tools.

This helps prevent non-tool services from enabling other
accessibility services.

Fix: 282987827
Test: Use a non-tool accessibility service to print the nodes on the
      screen, observe dialog nodes are missing. Change to a
      tool-service and repeat; observe nodes are present.
Test: Confirm that TalkBack can still use this dialog to enable other
      services.
Change-Id: I75c4101c6554cc503ed45fc24b8eb36d07250226