Added option (-z) to generate a zipfile with the bugreport.

Currently, dumpstate generates a text file and Shell zips it, but that
breaks the android.intent.action.BUGREPORT_FINISHED contract.

Also changed some string variables from char* to std::string.

BUG: 25563823
Change-Id: I6e61798bf559fa9f79f629b22fd410752da2bbf7