Add type to DisplayViewport

Refactor DisplayViewport to treat internal, external, and virtual
display viewports more equally. Now stored in single array, and mLocked
contains the most up-to-date copy of these. Still enforce the
requirement that there can only be 1 internal and 1 external viewport.
We should remove this requirement in the future.

Bug: 111108021
Test: atest DisplayManagerServiceTest
Change-Id: I6fca9725f6e4180063c86f3c2bc34906ac1d0024