bionic: use the size of the file to determine property area size

On the reader size, don't assume that the property size is PA_SIZE,
read it from the size of the file.  Allows init to use a different
property size without recompiling statically linked executables.

(cherry picked from commit 285b42a0)

Change-Id: I074204e9e6591b35faf7c1c58fb11ec162aff7bf