REVIEW: [NEXUS-6566] Replace use of ApplicationStatusSource with Provider<SystemStatus>

https://issues.sonatype.org/browse/NEXUS-6566
http://bamboo.zion.aws.s/browse/NX-OSSF90-1

The purpose of this tech debt item was to replace read-only usage of ApplicationStatusSource with Provider<SystemStatus>. A related issue has been created to examine the ApplicationStatusSource class hierarchy and refactor/simplify it:
https://issues.sonatype.org/browse/NEXUS-6641
Providers.of(mock(SystemStatus.class)) probably cleaner.