JobScheduler: Use StringBuilder for job name generation

Improve job name generation by using StringBuilder instead of Java
string concatenation.

This change enhances readability and extensibility by using
StringBuilder for string building, makes the code easier to understand
and modify.

Bug: 354795473
Test: atest FrameworksMockingServicesTests:com.android.server.job
Test: atest CtsJobSchedulerTestCases
Flag: EXEMPT refactor
Change-Id: If7b3027918108c0508ed880f3ecdb5415b882315