Check isZoomSupported before any zoom ratio calculations.

Per API doc, isZoomeSupported should be checked before calling
getZoomRatios or getMaxZoom. Moving zoom calculation into
isZoomSupported check fixed crash in front camera with no zoom
support, where a HAL/fwk returns a null list of zoom ratios.

Bug: 17863266
Change-Id: I80b5f7a431e7bbe4d50e5d062628e38eda3fd0a6