Introduce a result class for IStatusBarService#registerStatusBar()

This is a preparation to fix an issue that IME cannot enable light
navigation bar on direct-reply.

The initial state of StatusBar client is provided as return values of
IStatusBarService#registerStatusBar().  However, the way for the
client to receive the return values is to rely on "out" parameters in
the Binder method as follows.

    void registerStatusBar(IStatusBar callbacks,
            out List<String> iconSlots,
            out List<StatusBarIcon> iconList,
            out int[] switches,
            out List<IBinder> binders,
            out Rect fullscreenStackBounds,
            out Rect dockedStackBounds);

The issue is that if I want to add a new parameter into
StatusBarManagerService to fix Bug 122439339, then it needs to be
notified back to the client via one of these out values, e.g. a new
element in "switches", which is hard to understand and maintain.

It'd be better to introduce a dedicated return object as follows.

    RegisterStatusBarResult registerStatusBar(IStatusBar callbacks);

This is a purely mechanical code clean-up.  Hence there should be no
behavior change.

Bug: 122439339
Test: presubmit
Change-Id: Ib1c0ae8f591ca09d0bce7a39f85ba57aad386e47