Set display size to all displays

- get all displays from display manager
- loop and store the density values indexed by unique ID
- if DisplayContent isn't available for a display, fall back to DisplayInfo

Bug: 197730930

Test: Open Settings, go to Display -> Display size and text -> Display size, change the value, fold/unfold the device, see that the value persists.
Test: atest DisplayContentTests
Test: atest DisplayWindowSettingsTests
Test: atest WindowManagerServiceTests
Change-Id: I3d6858a9b20798c2ad3b12a66db3484491682691