Combine image string char arrays into single array

Having one giant char array shared between all the strings saves memory
since it avoids the 12 bytes of array overhead per image string. Also
added substring finding based on prefixes, strings are added into the
array in reverse sorted length.

Image size goes from 11767808 -> 11100160.

Bug: 17643507

Change-Id: I2a7f177b40d0458d5c50640643d8f16b0030bdce