Remove unnecessary SystemApi annotation from EasyConnectCallbackProxy

The class is a private inner class which is not accessible from anywhere
outside of WifiManager but is annotated with SystemApi. The annotation
is failing android.signature.cts.api.AnnotationTest#testAnnotation.

Bug: 124210145
Test: run cts -m CtsSystemApiAnnotationTestCases
Change-Id: I0c38e39f3e1cf5e161fafad4c243ebe2531830bb