limit the verbosity of app crash failure message

- reporting full dropbox content for all detected crashes leads
  to overly verbose test failure message

Bug: 67002148
Test: http://sponge/4ff3ccb5-c455-45e2-9614-d3527e1f711d
Change-Id: Ic2cd2ba4ce2af0954f186b196c18cc76c19f2007