Return DISABLED_FOR_USER in IMEI availability if not admin user

The correct return when checking for the IMEI field availability
for non-admin users is DISABLED_FOR_USER.

UNSUPPORTED_ON_DEVICE should only be used when the setting is
unconditionally hidden as the device does not support the feature.

Bug: 395714454
Flag: EXEMPT bugfix
Test: atest ImeiInfoPreferenceControllerTest
Test: atest MobileNetworkImeiPreferenceControllerTest

Change-Id: I4ee89a84953993daad539d33d3a2e576540c580f