Fix the bspatch failure on an empty brotli stream

When we close the BrotliDecompressor, we check for
BrotliDecoderIsFinished(). This function may return false if we never
call BrotliDecoderDecompressStream(), leading to a failure in bspatch
in case of an empty brotli stream.

This CL fixes the issue by checking BrotliDecoderIsUsed() along with
IsFinished(). Since we have the compressed stream and the precomputed
uncompressed size in the patch; we need these checks to avoid unexpected
size mismatch.

Bug: 69472150
Test: unittest pass
Change-Id: Icf7b324836fa59fd3111ba91b03622215a0c8f5f