TalkBack user switch

If TalkBack (accessibility speach) is enabled, then
when a user switch is performed (by clicking on the
user switch buttons), the device will announce that
it is switching users.

Test: manual (enable TalkBack, create a Guest)
Bug: 151669162
Change-Id: Ifa4a7b13f9a423d1dfa55873879fce71ca3a1673