New user switcher dialog

This dialog will eventually replace the UserDetailView. Right now, it
uses the same adapter to display the content (which is not necessarily
bad).

To enable:

1. `adb shell setprop persist.systemui.flag_new_user_switcher 1`
2. `adb shell am crash com.android.systemui`

Missing:
  * Red lines (including landscape size).
  * Bring user circles up to spec (once it's finalized).
  * Launch animations (to and from dialog).

Test: manual
Test: atest UserSwitchDialogControllerTest UserDialogTest
Bug: 185360864

Change-Id: I903d111822d1958fb53c43932b2df1393c841c71