pixelstats: add collection of VIMON data

Lastmeal allows collection of VIMON data experienced by battery. This
change enables collection of those parameters.

VIMON represents voltage/current monitoring on the IFPMIC.  We
are capturing the last IFPMIC voltage/current pair.

Test: cmd stats print-logs && logcat | grep <stat id>
Bug: 334127860
Change-Id: I20aae01c2d2b6bff08599b78812a0c0c4e52cc96
Signed-off-by: Hiroshi Akiyama <hiroshiakiyama@google.com>