Only propagates work source when explicitly set.

- Explicitly set means calling setCallingWorkSourceUid,
restoreCallingWorkSource or clearCallingWorkSource
- We clear the work source when handling BR_TRANSACTION to make sure the
work source is not propagated even when AIDL is not used (i.e. when
Parcel#enforceInterface is not called)
- We also restore the original work source after the transaction to
handle nested calls.

Test: atest binderLibTest BinderWorkSourceTest
Change-Id: Ia3ade2d2c5ebde5b1de2573943969fcb0d02d441