Clarify getProfiles

* Updates the javadoc of getProfiles, and similar methods, to make it
  clear that they return all users in the same profile group (not just
  the user's profiles, but also its parent and siblings, if applicable).
* Update the name of isProfileOf() to make it clear that it's actually
  bidirectional
* Fixes a confusing oddity in isSameProfileGroupNoChecks() in which it
  would return false if the user has an empty profile group, even if
  the two users are literally identical. This seems unexpected and is
  just waiting for an error. It currently doesn't matter because
  * almost all callers already explicitly handled the equal-users case
  * the one caller that didn't, actually should have, although it didn't
    matter because full users cannot have quiet mode anyway.
* Effectively removes hasProfile(). This method cannot handle all
  users, so it needs to be significantly modified. But it is no longer
  ever called anyway (the caller is behind an old flag), so we make it
  clear that is is effectively removed.

Test: atest UserManagerTest UserManagerServiceTest
Bug: 390707384
Flag: EXEMPT bugfix
Change-Id: I51b9f846ad61de8a37efe47a4645013c95996fb3