Reduce bug report compression level from 9 to 6.

This change reduces the zlib compression level for bug reports from 9 to
6, which will decrease deflation times at the cost of increasing
compressed bug report sizes.

Averaged over 5 back-to-back bug reports:
- Bug report deflation time decreased from 4.691s to 1.565s.
- Compressed bug report size increased from 12.82MiB to 14.60Mib.

Test: Build; check that bug reports are still taken correctly.
Bug: 215574756
Change-Id: Iedf2bf9f202ea6e0f7316c16da9abfdc14404701