binder: Fix ABI incompatibility between older and newer compilers

Parcel::writeString16's first parameter was a char16_t - defined to
uint16_t somewhere else.

With ISO C++11 compatible compilers, however, char16_t is a
compiler-defined-type that is not necessarily unsigned.

This causes the symbol to be different, breaking support for talking
to binary blobs built with pre-ISO C++11-compilers, such as the
Galaxy Nexus RIL.

Change-Id: Ibaa57fbd7d9fc4a95649d3b45d28d29963a9548b
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>