Do not report native memory metrics for isolated processes

Bug: 375101140
Bug: 375154337
Bug: 375106132
Flag: EXEMPT bugfix

We are not going to report native memory metrics for isolated processes
until they become significant, and thus there is no need to read aconfig
flags at all.

This will avoid the crashes in the bugs, while still keep using the flag
that's read-n-writeable. And also this avoids the need for a readonly
version of the same flag.

Change-Id: Ie1ef8e546e1edc546b1ff7ccc10b539586edbdbf