Fix robolectric test error

jobService is moved to mainline module. It is not registered in
the system at the beginning, therefore getSystemService returns
null. Instead of getSystemService, directly create a JobSchedulerImpl
and mock getSystemService method to return the JobSchedulerImpl
instance.

Fixes: 138714772
Test: make RunSettingsRoboTests
Change-Id: If91b6a63c0de92530d69f7b80515669352aab760