Return Color.TRANSPARENT from getNavigationBarColor

If an app is forced to go edge-to-edge, the APIs to set navigation bar
color are disabled. It should always get Color.TRANSPARENT from
Window#getNavigationBarColor.

Bug: 309578419
Flag: com.android.window.flags.enforce_edge_to_edge
Test: atest WindowPolicyTests
Change-Id: Ie8090db1f11160086930f9d1c4728a7d097d9eda