Rename the variable 'quality' to 'brotli_quality'

Right now the quality is only used in the brotli compressor. And it
would be ambiguous if we add another compressor in the future; since the
compressors may have different interpretion on the quality value.

Test: unit tests pass; run bsdiff
Change-Id: I23af06a135b2b47df5209171f7db0773ac02a326