Cache UserManager#isUserSwitcherEnabled

This IPC was happening every frame, which is a really bad idea :)
Now it's cached, such as it happens when resources change, or the
keyguard becomes visible.

Bug: 214567637
Test: atest KeyguardStatusBarViewControllerTest
Change-Id: I6afe8b794288bc5bce25c993a67f2ecc928d90a1