Enable brotli_bsdiff in update package generation

Brotli_bsdiff uses brotli to compress the bsdiff patches; and almost
always yields a smaller result than the old bsdiff with bz2. So there
is little point to try both methods in update_engine.

Example: walleye 4504078 -> 4585723 with bsdiff cache enabled.
               time      size
bz2 only:      ~8min     494M
brotli only:   ~12min    455M
bz2 & bro:     ~16min    454M

In this example, trying bz2 merely saves ~1M at the cost of 4 minutes
generation time. So in this CL, we only use brotli_bsdiff if the operation
is allowed; and later we can optionally implement a new mode in bsdiff
to pick the compressor who generates the smallest patches.

Bug: 34220646
Test: Generate and verify a walleye package
Change-Id: Ideae1acea95ff204c2ba5b7637945307743f0a51

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