docs: Clarified when GET_ACCOUNTS is necessary

Per bug report, clarified when an app needs GET_ACCOUNTS permission
to read the current account's information.

Along the same lines, clarified that the CONTACTS permissions group
regulates permissions for all accounts and contacts on the device, not
just the user's contact info (since GET_ACCOUNTS is in CONTACTS, and,
as noted, GET_ACCOUNTS applies to other users' info).

See first comment for doc stage location.

bug: 23589012
Change-Id: Ia87565936787dd85daedfb641883e7cbf778f2bd