Enable error-prone checks by default

Enable error-prone checks by default

Based on recent work by Dave Borowitz on the Gerrit project [1, 2].

The warnings/error configuration is unchanged, but now the checks are
enabled by default during the build rather than having to be manually
invoked.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/225653
[2] https://gerrit-review.googlesource.com/c/gerrit/+/225714

Change-Id: I7f4849896af72de38880b13d64519cc93bb3d2a3
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>