[Telephony] Add description about location permission

    - Hold ACCESS_FINE_LOCATION: full service state
    - Hold ACCESS_COARSE_LOCATION: set cellIdentity is null
    - No local permission: set cellIdentity, mOperatorAlphaLong, mOperatorAlphaShort
        and mOperatorNumeric are null

Bug: 165014401
Test: make
Change-Id: I9ada08a530d9b8bc8483edfad783ff3b7434faa3