stagefright aacenc: Mark codeWord as UWord16

Without this, the codeword 0xffff will be expanded to
0xffffffff when passed as a Word32 parameter. All code words
set into this variable are UWord16 initially.

When it was expanded to 0xffffffff, it could accidentally
overwrite earlier cached bits in the bitstream writer.

Change-Id: Id91a2b8a4e934876d9cb39eaeeee323202ec8e40