Hide ContentInfo.partition from the public API

Per API Council feedback, this method will not be exposed in the public
API. App developers will instead use the support library for this.

Rather than removing the method completely, it is marked with @hide so
that it can still be used in system code (e.g. RemoteInputView).

Bug: 174125311
Test: atest CtsViewTestCases:ContentInfoTest
Change-Id: I83b55bea38c0d2b3dc60a16658616e46ebe4cb48