Allow blocking calls when protected against ANRs.

ContentProviderClient has a nice setDetectNotResponding() method
that detects hanging calls to remote providers, and it can trigger
an ANR to kill the app and release the blocked thread.

We typically don't want to perform blocking calls from the system
server, but we're okay allowing them on CPCs that are using
setDetectNotResponding() to watch for hung clients.

Test: builds, boots
Bug: 69128093
Change-Id: I223aaf1d0cef0f8dee28f800d9e3c101d7449952