Change users without crashing a11y services.

Accessibility services crash when changing users for several
reasons. we were looking up permissions for the current
user, which wasn't the user who created the service. We also
end up resetting the accessibility state, which reads user-
specific settings and thus must be done with the calling
identity cleared.

Bug: 27594523
Change-Id: I2b910d77704a6054b3a591c38f54d3ed3a2dc427