libcamera: bump postview sizes

* @pawitp noticed that libcamera uses PostviewHeap to store
  temporary YUV data, which is not large enough for
  VGA selfies
* Double the values

Change-Id: I139655bd12d752326a8237a86305b31c23fd0cad
libcamera: Remove uncommon resolutions

Removing the strange resolutions (1.5 MP and 2.5 MP) from the camera HAL as
they don't seem to work properly (weird preview rendering and FC's while
taking snapshots)