Reduce maximum size of strings in AutomaticZenRule

Halving these should approximately double the number of (maliciously crafted) rules that the system can handle.

Fixes: 387498139
Test: atest AutomaticZenRuleTest (preexisting)
Flag: EXEMPT Simple bugfix
Change-Id: Ib62e669192d25fa435f38537b703ba9f6291e0d4