Refactored zipfile generation.

Previously, the bugreport.zip was only created at after dumpstate
finished, at which point the temporary file was added to it.

With this refactoring, the bugreport.zip is created earlier on and the
temporary file is added at the end: although this change doesn't alter
the final result, it allows future changes to add more files to the .zip

BUG: 26293568
Change-Id: Ic0a111d009aac954c9746130df226a2dfeb679bc