Attach timestamp to ringbuffer generated files

The file last modified time expressed in seconds from epoch is
attached to only the archived version of the file that is passed
to dumpstate.
Files stored on the device will still have no timestamp in their name.

Bug: 159808285
Test: manual verification
adb root
adb shell lshal debug android.hardware.wifi@1.5::IWifi >> archive.cpio
cpio -iv < archive.cpio
Verify that a timestamp is attached to the filename at the end:
connectivity_events_rbhOVjYpDdNY-1597364258

Change-Id: Iee0c2b37fc1d27cb979ec9125461416cd2d02549