Camera: Avoid possible NPE after extension session configuration

Camera extension clients are able to close and release camera and
all associated extension resources while an advanced extension session
configuration is in progress.
In case, the extension initializer callback gets executed after all
resources are released it may try to access the already released
binder interface triggering NPE.

Bug: 268437036
Test: atest -c -d
cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionSessionTest.java

Change-Id: Ib6fdc9d08e973616dc7d83264dc3f320e496cbd3