Clarify the meaning of weight in the focus and metering area HAL.

Weigh values must be less than 1000, but have no restriction on their
sum. Weights should be interpreted as per-pixel values, meaning larger
areas will have more influence over focus/metering than smaller areas
of the same weight.

Change-Id: I988b74f988ddba2ff0c49d078ff3174b47cbae18