Added getProfileIds method returning array of userIds

Previously many usages of UserManager.getProfiles and getEnabledProfiles
were only using ids of returned users. Given that the list of users needs
to be parceled and unparceled for Binder calls, returning array of ids
minimizes memory usage and serialization time.

A new method getProfileIds was introduced which returns an array of userIds.
Existing method calls were updated where appropriate.

Bug: 27705805
Change-Id: Ib042f38d53d95d3b07cda7f824e5cb8c06cd10f5