Let the dimming window can provide multiple appearance regions

A dimming window which extends to status bar area can divide status bar
into different appearance regions (up to 3). For example:

+---------+-------------+---------+
|/////////|             |/////////| <-- Status Bar
+---------+-------------+---------+
|/////////|             |/////////|
|/////////|             |/////////|
|/////////|             |/////////|
|/////////|             |/////////|
|/////////|             |/////////|
+---------+-------------+---------+
     ^           ^           ^
 dim layer     window    dim layer

This CL allows non-fullscreen dimming window can specify its appearance,
and creates appearance regions in the window bounds, so that status bar
can adjust the foreground color in each regions.

Before this CL, it would create only one appearance region in the case
above, so the status bar icon color might not be displayed properly in
some regions.

Bug: 208717544
Test: Show a dimming window which extends into status bar area.
Change-Id: Ie81ce19e9a504045988c4852795117a15460b709

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