Add type to DisplayViewport

Add type to DisplayViewport. Store a single array of viewports, but
only allow 1 internal and 1 external viewport for now.
DisplayManagerService now totally in charge of the viewports.

Bug: 111108021
Test: atest -a libinput_tests inputflinger_tests
Change-Id: Iac3dbc2e4a61b9609344db475d29bcdc7c167b1c