Check multi-user support in isUserSwitcherEnabled

isUserSwitcherEnabled now returns false if multi-user is disabled(
supportsMultipleUsers() returns false).

Removed GUEST_USER_ENABLED setting and replaced with DPM.getGuestUserDisabled
check. It currently always returns false, but will be replaced with an actual
policy check.

Bug: 17571233
Change-Id: I41853e8b321b2537952cac5d92e88bfdb8cbd9a8