Sets dumpstate version using system properties.

Android N introduced a '-V' argument to change dumpstate's version, but
such argument can only be used when running dumpstate directly (as
root).
This CL uses a system property instead (dumpstate.version), so different
versions can be used while taking bugreports.

It also introduces a '-v' argument line to display the dumpstate header,
and removes the redundant dry_run check from _dump_file_from_fd().

Test: DumpstateTest passes
Change-Id: I46ec4bca133d16d8431358bb9f16eb89bb082be1