Clear the Parcel before writing an exception during a transaction

This prevents any object data from being accidentally overwritten by the
exception, which could cause unexpected malformed objects to be sent
across the transaction.

Test: atest CtsOsTestCases:ParcelTest#testExceptionOverwritesObject
Bug: 34175893
Change-Id: Iaf80a0ad711762992b8ae60f76d861c97a403013
Merged-In: Iaf80a0ad711762992b8ae60f76d861c97a403013