ril: Fix build with ISO C++11 compilers after ABI fix

Fix implicit signed-to-unsigned cast after restoring
the old ABI for android::os::Parcel::writeString16

Change-Id: I6732fe2e4f2b45d636bf59278ed98efa979c4393
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>