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
(cherry picked from commit 86441bd5c07c65021728748472fa6a28eaf162d0)
(cherry picked from commit 0c94ea393926869f8f620854b58548d7a139ac1f)
Merged-In: I3f3a99765c161369e1b026686a0e5f0c83ed839e