Fix visibility and editability of importance fields

- Block field should always be visible
- Locked by OEM: cannot block or change importance
- Locked by default app: cannot block, can change importance
- Locked by system app: cannot block, can change importance
- system app but blockable: can block, can change importance

Test: robotests
Fixes: 131248127
Change-Id: Ifa718c84573dd5125aefa4f672a79dc4f267d515