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>
Merge remote-tracking branches 'linaro/linaro_android_4.2.2' and 'rqkang/cm-10.1' into cm-10.1