Disable showSpn under no service condition

"No Service" with SPN text is displayed on lock screen under no service
condition. But most of the operators require not to display SPN text on
that condition since showing "No Service + SPN" confuses user. By
disabling showSpn, SPN text is not displayed on lock screen.

Test: Verified by manual
Bug: 31952303
Change-Id: Ieea7172271ab0f8c179b6963201faee7c2c7882d