Make DevicePolicyManager.isAffiliatedUser public

- Simplified javadoc on device affiliation concept. "affiliated with the
  device" will be used to describe affiliation with device owner user,
  as defined in #isAffiliatedUser javadoc.
- Link to #isAffiliatedUser whenever device affiliation is mentioned

Bug: 67582080
Test: None
Change-Id: I4395ff980aa1626d6ff352d2f16777887784082c