Pass int32_t for JNI calls to java Input/Output streams.

Passing size_t is problematic on 64 bit platforms where
it's 8 bytes in size. Conversion to int32_t is safe because
the size argument is always clamped to fCapacity, which is
4 bytes wide.

Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Change-Id: I58558561a4f56451485f1a5fc6cdeda677247071