Cache correct calling UID for VcnStatusCallbacks.

This CL updates VcnManagementService to cache the correct UID for the
caller when a VcnStatusCallback is registered. Previously,
getBinderCallingUid() was checked after the calling identity was
cleared.

Bug: 183437200
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: Idfa2d2aad6bc03b3d08a137f9913276057632a3b