Disable multi-user background recording

On user switch, kill existing processes of the background user with
android.permission.RECORD_AUDIO permission. Home activity should not be
killed to avoid an expensive restart of the home launcher, when the
user switches back.

Introduced DISALLOW_RECORD_AUDIO user restriction, which is enabled for the
background user, and removed for the foreground user.

Introduced a concept of system controlled user restriction, which can only
be set by the system, rather than device administrator.

Bug: 20346194
Change-Id: Ic942fd565e80d14424230dae612965a8e229c4ef