Upload tombstone with the system_server_native_crash tag.

This allows monitoring system server native crashes separately from
all the other native crashes.

Note that this now double-reports native crashes in system server,
once with SYSTEM_TOMBSTONE tag and once with the
system_server_native_crash tag. This can likely be cleaned up to
only report the latter, once it's been verified to work.

Test: "ls /data/system/dropbox" after triggering a native SS crash.
Bug: 63172851
Change-Id: Ib09275b53627c0219e2f74ea2ab102575f9b33c0