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Calculates the memory cache size based on a percentage of the max available VM memory.
Eg. setting percent to 0.2 would set the memory cache to one fifth of the available
memory. Throws
IllegalArgumentException if percent is less than 0.05 or greater than .8.
memCacheSize is stored in kilobytes instead of bytes as this will eventually be passed
to construct a LruCache which takes an int in its constructor.
This value should be chosen carefully based on a number of factors
Refer to the corresponding Android Training class for more discussion:
http://developer.android.com/training/displaying-bitmaps/