Put syncs in a dedicated job namespace.

Put all sync jobs in a dedicated SyncManager job namespace so that sync
jobs don't conflict with jobs scheduled by other parts of the system.

Bug: 238764917
Bug: 253649458
Test: atest CtsSyncManagerTestsCases
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/content
Change-Id: I65db3933b1622353401f86a95d770097c45aebbd