Allow IME windows to use light navigation bar

With this CL, a visible IME window that has
FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS is officially allowed to use
SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR like a typical fullscreen
application window that has SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR, as
long as FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS is set to the IME window and
there is no dimming window [1] is shown above the IME window layer.

Note that an example of not setting FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS
for IME windows is a flotating IME, where IME window is completely
disjoint from the navigation bar region, like a typical floating
window.  In this scenario, the navigation bar background/button color
should be determined by the target application's flags.

 [1]: A window that has FLAG_DIM_BEHIND flag
 [2]: I4b10a19641bd3ce6c43e7629404b6f202d4186e8

Bug: 69111208
Fix: 69002467
Test: atest android.systemui.cts.LightBarTests
Test: atest com.android.server.policy.PhoneWindowManagerTest
Test: atest CtsInputMethodTestCases
Test: Manually tested with ThemedNavBarKeyboard sample [2]
Change-Id: I40d79f364059d13a7748f4935db17edebaf7ba2d