Don't use MockIContentProvider

MockIContentProvider is not part of the public API of android.test.mock.
This test has been able to use the private API directly because it is
with 'platform_apis: true' and thus was provided with the impl lib
of the mock library. However, this will be prohibited when the mock
library is built with 'default_to_stubs: true' to hide the impl lib.

In preparation for the change, let the test implement the mock
directly from IContentProvider using Mockito.

Bug: 157007292
Test: m
Test: atest FrameworksUiServicesTests
Change-Id: I6de7f880baa19eebbc46076f6402173c68c44f8d