(1/n) Phase out FEATURE_ONE_HANDED_BACKGROUND_PANEL

Remove DisplayArea feature FEATURE_ONE_HANDED_BACKGROUND_PANEL
We are going implement One handed panel throught
WindowlessWindow mechanism instead.

Bug: 197165590

Test: manually test One-handed mode basic operations.
Test: atest WMShellUnitTests
Change-Id: I3ba106a266e69bf61cdd43c0043852eed14dc365