Initial splitting of Content Providers from AMS.

This is the first of many CLs for refactoring Content Provider related
code from Activity Manager Service.

This CL simply pulls out most of the CP code from AMS into its own
class. There are no logic or lock-related changes in this particular CL.

Bug: 156262145
Test: atest ContentProviderTest [all]
Change-Id: I03310a1f06d354e15439a37f7aeffa543192dad3