Rename "legacy" stop reasons.

There are some uses for the "legacy" stop reasons that the public API
stop reasons can't cover well. Since we'll be keeping the "legacy"
reasons around, renaming them to "internal" to help explain the
differentiation.

Bug: 141645789
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest ContentResolverTest (all)
Test: atest CtsJobSchedulerTestCases
Test: atest CtsSyncManagerTest
Test: atest SyncManagerTest
Test: atest SyncOperationTest
Test: atest SyncRequestTest
Change-Id: I97e40d35b79dbd012216bc22e90d649639614854