Use readonly version of aconfig flag native_metrics

Bug: 375072865
Flag: EXEMPT bugfix

This has the same root cause as b/368409430. Use the readonly
version of aconfig flag native_metrics, so there will be no
I/O involved in Isolated Processes which do not have permission
for I/O.

Change-Id: Ib4d58e47a7b7244f55b7bd2c3d4d5241187875e7