base: Fix build after Parcel ABI fix in Binder

Adapt code to the fact that writeString16's first parameter
is now an uint16_t* to match the old ABI

Change-Id: I138af645c877f343f0490152b9702698b7d36582
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>