libcamera: Bump front camera postview sizes

* @pawitp noticed that libcamera uses PostviewHeap to store
  temporary YUV data, which is not large enough for
  1.3 MP selfies.
* However, bumping the values broke rear camera, which
  @bbelos noticed.
* Fix that by bumping the values only for the front camera.

Also-By: Pawit Pornkitprasan <p.pawit@gmail.com>
Also-By: Brian Beloshapka <bbelos@hotmail.com>
Change-Id: I139655bd12d752326a8237a86305b31c23fd0cad
Libcamera: Adjust postview for the front camera (VGA quality)