Modify Process::readProcFile to read in variable size file

The original Process::readProcFile could only read files smaller than 256 bytes.
This change allows the function to read in larger files. For files smaller than
256 bytes, the added cost is only an extra read call, which returns zero.

Documentation has also been added to Process::readProcFile.

Test: Used in change 5168194

Change-Id: I82f27355276e08f96bdb5fe7da49514a87afb429