Properly handle HSUM no-user in the ContextHubService

When the system is in Headless System User Mode (HSUM),
the system runs the UI as user 0. User 0 cannot have
a credential and the device is automatically unlocked.
Any user can change the microphone setting.

Bug: 278961972
Test: Run the MicDisableSettingsTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:8ff589c9120113b2b61c27accd319cd04cef5980)
Merged-In: I1d24b1cfe2222868670e4b8569229f1ed0ad378b
Change-Id: I1d24b1cfe2222868670e4b8569229f1ed0ad378b