camera2: Fix null pointer exception while initializing max zoom ratio

Before initializing the max zoom ratio, we should check if the zoom
feature is supported by the currently used camera.
Some old legacy front cameras don't have zoom feature and the camera
app, without this commit, throws an error because of the null pointer
exception.

Change-Id: I479ed6a33b8aba1ab2219d409fcd1b35179d0ee0