Get gravity of menu panel by display

Also:
 - Reorganize import of DisplayContent and PhoneWindow.
 - Fix DisplayContent leakage in wm test.
   (SurfaceFlinger abort in createLayer -> addClientLayer with
    "Suspected IGBP leak: 4097 IGBPs (4096 max)")

Bug: b/111362047
Test: atest FrameworksServicesTests:DisplayContentTests# \
      testGetPreferredOptionsPanelGravityFromDifferentDisplays

Change-Id: I9c5a0f59a86fda08bd532cb2b66bab76b2dfa6c8