Allow for multiple internal and external viewports.

Also, don't restrict to non-default displays for internal viewports;
leave that up to the input system to decide the policy on selecting
which display to use if there's more than one.

Bug: 116824030
Bug: 171549575
Test: atest InputReader_test
Change-Id: I0f29b4c6043edf4f3e8fa5ac10380d67b11203d3