Use char16_t instead of int in FilterClient to pass 16-bit data defined in TunerHAL

char is char16_t in cpp backend when defined in AIDL interface.
Which matches uint16_t type defined in the Tuner HAL.
See: https://source.android.com/devices/architecture/aidl/aidl-backends#types

Test: make
Bug: 185627837
Change-Id: I287197c34ef5a892c05df3cb9226006fd2cf88dd