wpantund: Allow wpantund to call callbacks

This change allows wpantund to call any binder callbacks that have
been registered with it. Generally, only privileged apps are allowed
to register callbacks with wpantund, so we are limiting the scope for
callbacks to only privileged apps. We also add shell to allow the
command-line utility `lowpanctl` to work properly from `adb shell`.

Bug: b/67393078
Test: manual
Change-Id: I64c52cc5e202725a81230dc67e1cd7c911cf8e1c
(cherry picked from commit 17319cb3)

public/wpantund.te

@@ -10,6 +10,13 @@ binder_call(wpantund, system_server)
 # wpantund needs to be able to check in with the lowpan_service
 allow wpantund lowpan_service:service_manager find;
 
+# Allow wpantund to call any callbacks that have been registered with it.
+# Generally, only privileged apps are able to register callbacks with
+# wpantund, so we are limiting the scope for callbacks to only privileged
+# apps. We also add shell to allow the command-line utility `lowpanctl`
+# to work properly from `adb shell`.
+allow wpantund {priv_app shell}:binder call;
+
 # create sockets to set interfaces up and down, add multicast groups, etc.
 allow wpantund self:udp_socket create_socket_perms;