Use @CriticalNative annotation on Process.getElapsedCpuTime()

This method runs in bounded time and doesn't need a JNI Env,
so it's safe to annotate it as such.

Bug: 374158990
Flag: EXEMPT annotation change can't be flagged
Test: presubmit

Change-Id: I903d71727f291f3f6b45d56d04f81373351836f2