Set a good example - don't export ExamplePlugin

Plugins should never be exported because this would allow other
packages installed on the device to attempt to launch it via
Context#startService which can crash SystemUI.

By default, if a service has an intent-filter, exported is set to true.
Therefore, we must explicitly set exported=false.

Test: n/a
Bug: 142012333
Change-Id: Iaf4191ee3006cc07eb0309f068854c103683e99d