fg-alg: Add TTF support for step charging

Step charging configuration comes from the battery profile which
defines the variable charging current configurations based on
Vbatt or State of charge (SOC). When voltage based step charging
is enabled, Time to full (TTF) calculations needs a change as the
charging rate changes. As per the hardware recommendation, add
the calculation needed to go through each of the charging window
and compute the time.

Change-Id: I9b264b8af25f93aba20a8609f4786427081764a0
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>