Pass in the source buffer size depend on the input file size

the xdelta might produce huge output if the difference is bigger than
32MB, half of the default buffer size 64MB.[1]

xdelta author suggested nearest power of 2. [2]

[1] http://code.google.com/p/xdelta/wiki/TuningMemoryBudget#Source_buffer_size
[2] http://code.google.com/p/xdelta/issues/detail?id=175

Change-Id: Id4f277373d088986fc314c386b269d0bddfcdd07
Merge "Pass in the source buffer size depend on the input file size" into android-4.4