Fix the endless panel loading

Re-launching volume panel continuously will trigger an endless panel
loading, show a transparent unfinished UI, and then block the user's
screen.

Root cause:
When the activity receives a new intent from user's clicking, it will
call PanelFragment#createPanelContent to update the current fragment.
The method triggers an animation and then loads the panel content. If
multiple invocations run concurrently before the animation or the
loading finish, the loader's countdown latch will be increased
abnormally and lead to the endless loading.

Solution:
1. Since the invocations are in UI thread, simply add a flag to avoid
reentrance when the panel is animating or loading.
2. Filter out the same panel's creation request when the panel is still
visible.
3. Do not force a panel's recreation when it's under construction.

Fixes: 143889510
Fixes: 160491854
Test: robotest, manual
Change-Id: I821faedeb62354929f3af9804cbbe44ee5bb8a53
Merged-In: I821faedeb62354929f3af9804cbbe44ee5bb8a53
(cherry picked from commit 6a8d2c5e)