Return null for contacts local account

In T+ the local account is required to be consistent with the what is
used in the AOSP ContactsProvider and so
RawContacts.getLocalAccountName and RawContacts.getLocalAccountType
should always return null.

Test: atest CtsContactsProviderTestCases
Bug: 188347648
Bug: 197121153

Change-Id: I3b7143e3b96e5db53128f4b69b0eb606237f9f50