This CL moves rotation tracking from WindowManagerService to
DisplayContent. This way displays can be rotated independently and
rotation of the main display won't affect rotation of secondary
ones.
Bug: 34242678
Test: android.server.cts.ActivityManagerDisplayTests
Test: testRotationNotAffectingSecondaryScreen
Change-Id: Ic46aaa523482b31ff5ec77f0c2908ceda1156fc0