Reuse buffer when reading bytes from files.

Previously AppFuse getObjectBytes returns byte array and the array's
length needs to equals to the exact number of bytes the method read.

The CL change the function signature so that it can return the number of
read bytes. And reuse a buffer array instead of slicing the array with
valid length.

BUG=None

Change-Id: I78b714554cac9ae71b895cb8929bc98969f5a8ca