Gallery2: fix build

mNext and mPrev have private access, yet they are accessed from outside
of the class. Mark them as protected to fix this.

    packages/apps/Gallery2/src/com/android/gallery3d/util/LinkedNode.java:59:
    error: mNext has private access in LinkedNode
                return (T) (node.mNext == mHead ? null : node.mNext);
                                ^
    packages/apps/Gallery2/src/com/android/gallery3d/util/LinkedNode.java:59:
    error: mNext has private access in LinkedNode
                return (T) (node.mNext == mHead ? null : node.mNext);
                                                             ^
    packages/apps/Gallery2/src/com/android/gallery3d/util/LinkedNode.java:63:
    error: mPrev has private access in LinkedNode
                return (T) (node.mPrev == mHead ? null : node.mPrev);
                                ^
    packages/apps/Gallery2/src/com/android/gallery3d/util/LinkedNode.java:63:
    error: mPrev has private access in LinkedNode
                return (T) (node.mPrev == mHead ? null : node.mPrev);
                                                             ^

Change-Id: I6c05b9a9edda7a4e7bce3c632a3e662b9b303645