Add bound checks to utf16_to_utf8

Bug: 29250543
Change-Id: I518e7b2fe10aaa3f1c1987586a09b1110aff7e1a
(cherry picked from commit 7e93b2ddcb49b5365fbe1dab134ffb38e6f1c719)

size_t Parcel::dataAvail() const
{
 size_t result = dataSize() - dataPosition();
 if (result > INT32_MAX) {
        abort();
 }
 return result;
}

size_t Parcel::dataAvail() const
{
 size_t result = dataSize() - dataPosition();
 if (result > INT32_MAX) {
        abort();
 }
 return result;
}
