Narrow access to getDataLayerSnapshotForUid().

This method is used in TrafficStats to obtain the snapshot for the
calling UID, so narrow it to only return data for that specific case.

We might be tempted to throw an exception or adjust the method
signature, but apps are using the raw AIDL in the greylist, so we
return them an empty NetworkStats if they ask about other UIDs.

Bug: 119672472
Test: manual
Change-Id: I3064630039f1d8473d6425dd663f17813a4ae3a1