Add a readonly version for flags in libcore

Bug: 368409430
Flag: EXEMPT bugfix

The exported flag native_metrics from com.android.libcore breaks
some use cases of Isolated Process as reading of the flag requires
I/O permission which Isolated Processes don't have.  In this CL,
we added a readonly version of the flags in libcore, and use
`force-read-only` mode, the resulting generated code will return
the values hardcoded. This prevents reading of the flag value
from storage, and thus allows Isolated Process to work.

Access to the flag will be done by:

  com.android.libcore.readonly.Flags.nativeMetrics()
  com.android.libcore.readonly.Flags.postCleanupApis()

This readonly flags will be kept to be synced with the one in libcore.
Once the new aconfig API is landed, there will be no need to keep the
readonly version, and thus this CL can be reverted.

Change-Id: If4c56dae63be879b54f951bb8c7904ca3b98607e