Move PhonePolicy to Optional

Despite the @Nullable and the comment, we keep forgetting to check for
nullness on mPhonePolicy.

By moving the field into an Optional, this need to be check by design
and will prevent any further crash.

Bug: 380316229
Test: m .
Flag: Exempt refactor
Change-Id: I98b24e0c6a5419802396743d8c01e4b74f849d99