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