Bug 1037097: restrict image decode size in MediaFrame based on device memory size

@davidflanagan -- sorry, i couldn't find the code which does runtime detection of the available RAM. If you are going by /proc/meminfo then please note that the available MemTotal will be smaller than the provided RAM size.
e.g., 181MB for total RAM size of 271MB which will put it in <256 category instead of the > 256 and < 512.