Fix drag and drop URI permission grants

Currently cross app drag and drop with content URIs
only works for Owner. This is because the source Uid
is initialized after a Binder.clearCallingIdentity call.
The real source app identity is lost and things work
by accident for Owner only.

Bug: 25232077
Change-Id: Iaaaa735a04909a3b05fae7a7a421dbf02206cd68

services/core/java/com/android/server/wm/DragState.java

@@ -77,7 +77,6 @@ class DragState {
         mSurfaceControl = surface;
         mFlags = flags;
         mLocalWin = localWin;
-        mUid = Binder.getCallingUid();
         mNotifiedWindows = new ArrayList<WindowState>();
     }
 

services/core/java/com/android/server/wm/WindowManagerService.java

@@ -6970,7 +6970,7 @@ public class WindowManagerService extends IWindowManager.Stub
                     + " asbinder=" + window.asBinder());
         }
 
-        final int callerPid = Binder.getCallingPid();
+        final int callerUid = Binder.getCallingUid();
         final long origId = Binder.clearCallingIdentity();
         IBinder token = null;
 
@@ -6995,6 +6995,7 @@ public class WindowManagerService extends IWindowManager.Stub
                         final IBinder winBinder = window.asBinder();
                         token = new Binder();
                         mDragState = new DragState(this, token, surface, flags, winBinder);
+                        mDragState.mUid = callerUid;
                         token = mDragState.mToken = new Binder();
 
                         // 5 second timeout for this window to actually begin the drag