Lower per-app NotificationChannelGroup limit.

A higher limit can be abused to cause PDoS.

For the rationale behind choosing 6000 as the max value, see
http://b/261723753#comment20.

Test: atest PreferencesHelperTest
Bug: 261723753
Change-Id: I3f3a99765c161369e1b026686a0e5f0c83ed839e