Greylist test API for CtsSliceTestCases

The test invokes two hidden methods inside a ContentProvider.
Since we cannot exempt those, add the two methods to light greylist
until they are made @TestApi.

Bug: 64382372
Test: run cts --module CtsSliceTestCases
Change-Id: I9bc0c23282c1744f4b93b9d84fcdd022f70859c2