Camera2: Calculate UI aspect ratio based on preview size

- This reverts change If4047860.

- Simply using preview size instead of picture size
  at all to calculate the UI aspect ratio,
  avoiding tricky overwriting aspect ratio.

- Will fix issues when surface texture transform
  is calculated based sometimes on preview size
  ratio and sometimes on picture size ratio.

CRs-Fixed: 601495
Change-Id: I8e054c9eaafc9fb54dac2bff2c67e39f0a1fe64c