Exposed some testAPIs in UserManager

The following changes in UserManager are required for ongoing
improvements to CTS/GTS infrastructure.
- Exposed the following as testAPIs:
	* isGuestUserEphemeral
	* createUser
	* preCreateUser
	* createProfileForUser
	* createRestrictedProfile
	* getUsers
	* getUserType
- Added permissions to the following APIs:
	* CREATE_USERS for getUserName
	* INTERACT_ACROSS_USERS for getProfileParent
- Added CREATE_USERS permission to the documentation of
getUsers and the APIs that call it.

Test: Confirmed that a CTS test can call these APIs
Bug: 178695365
Change-Id: Id9105db3bfefb926139715b728acedefd2a5ca3e