libasync_safe: stop clobbering other folks' identifiers.

The log priorities and ids are in an NDK header, available to everyone.

Move CHECK into its own header for now. This would be better if it was
more like the <android-base/logging.h> CHECK family, but I don't have an
easy way to do that without lots of copy & paste, so punting for now.

Bug: https://issuetracker.google.com/issues/119713191
Test: boots
Change-Id: I4566be8a0a024fede0e2d257c98b908ec67af2a8