camera2: Mark onCameraOpened/Closed callbacks @SystemApi.

This is so that regular apps don't get to know when camera devices
are opened / closed (as oppsed to being available / unavailable), in addition to not knowing the
package names of the processes that open / close the devices.

Bug: 147710835
Bug: 170870941

Test: builds
Test: atest CameraManagerTest#testCameraManagerListenerCallbacks

Change-Id: I5cad3cb5dd70775d74c5be20d7adf434fd37bfa7
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>