power: vm-bms: Add UUC (unusable capacity) filtering logic

Use a 16 sample average of IBAT for UUC calculation. This
smooths out the UUC and avoid jumps in SOC if IBAT changes
rapidly. Also, allow the UUC to change only by 1% across
every iteration.

Change-Id: I09430ebfb980d2fe4e9578dca0cd1dc22fcbbb12
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>