Use readonly flag of native_metrics for HardwareBuffer

Bug: 368409430
Flag: EXEMPT bugfix

In this CL, we used 'com.android.libcore.readonly.Flags.nativeMetrics()'
instead, this flag is forced-read-only which does not involve I/O, thus
preventing failures such as in "Isolated Process" where I/O permission
is disabled. See ag/29673659.

Change-Id: Ic48c26ba2810c405c7ca0ca93faf23ed04001336