Replace verbose @Provider with a private @Binds

The original version of the dagger code used a Provider so that the
private SensitiveContentCoordinatorImpl class was not exposed as part of
the public API signature (which would cause a compilation error).

This change utilizes a quirk with Dagger Module "includes" where you are
able to include a private Module inside of a public one. This private
module is able to use the symbols private to the Kotlin file (in this
case, SensitiveContentCoordinatorImpl).

The downside here is that the tests have become slightly more verbose.

Test: atest SensitiveContentCoordinatorTest
Change-Id: I06d87e68e5b40f432bc39d1015b54c03f141fc4d