[am] add place holders for new crash stats fields

Putting empty values for now to cope with proto changes. Will replace
the place holders with real values in follow-up CLs.

BUG: 184844615
Test: atest android.cts.statsdatom.incremental.AppErrorAtomTests
Change-Id: I5a6adcf91c67f331a43e76ee706fdb5f2ffe67d6