Set FailureRetention default snapshot number to 2

After more experiments and discussion we concluded that we should use a
smaller default maximum snapshot number because of the disk requirement
for snapshots. Thus change this number from 5 to 2.

Also update some comments to match the current behavior of Test Failure
Retention.

Bug: 177959321
Test: RetentionConfigTest
Change-Id: Ibedfee580012e6ba72682fe7fed2ed76c4105ad3
3 files changed