Cast HIDL uint16_t to char16_t in AIDL cpp backend service

Note that 16 bit int in AIDL interface is char type, while the
corresponding type in cpp/ndk backend is chat16_t type.

See https://source.android.com/devices/architecture/aidl/aidl-backends#types

This CL changes all the casting from uint16_t to char16_t instead of
char to keep the right size of the data.

Test: make
Bug: 185627837
Change-Id: Ic41c7d15df62adb4f78216580fcd9af2c806d891