camera2: Change android.lens.focusRange to Pair<Float,Float>

Since the focus range is defined in diopters (1/distance), it just so
happens that in a range [near, far] near >= far which is against the
android.util.Range contract.

Change it to pair instead.

Also adds a marshaler for Pairs since they didn't have one before
(and aren't parcelable).

Bug: 15335269
Change-Id: If2f3259aeec072438a1c18c1b9728a4c81e92a40