google_dual_batt_gauge: use capacity ratio to calculate reported soc

change average soc to weighted soc due to two batteries with different
capacity(size), the larger battery has higher weight.

Bug: 188768844
Signed-off-by: Jenny Ho <hsiufangho@google.com>
Change-Id: Ifdfdd1596742a2e3bbef013191f725d729fcc9f3