camera2 jni: nativeReadValues() can use camera_metadata_ro_entry instead of camera_metadata_entry

We don't guarantee CameraMetadata access is thread safe, however for
existing apps that might query camera metadata from multiple threads,
this change could help avoid read failures due to the need for native
camera metadata's internal lock.

Also return a NULL array instead of a 0 sized array in case the
entry.count is 0 since the caller expects either something with a valid
number of entries or null.

Bug: 371223465

Flag: EXEMPT bugfix

Test: manual GCA

Change-Id: I144f8e5438a4135440e34b9c65f326bba7748faa
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>