power: fg-alg: Show last TTF while clearing Ibatt samples

Currently, when the battery current (Ibatt) changes by more than
500 mA, TTF algorithm clears Ibatt samples so that Ibatt average
can be obtained with fresh set of samples. This was done to
improve TTF accuracy. However, while doing it TTF shown to the
user is shown as -1 because collecting Ibatt samples would take
few seconds. Fix this by showing last TTF to the user when Ibatt
samples are cleared until new set of Ibatt samples are obtained.

Change-Id: Ifba1d399f38893dd544be2369f600a7400530e64
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>