Support multiple internal and external viewports

The policy is still effectively the same by selecting the first display,
but this lets devices which are configured with a specific unique ID to
see the viewport that should be associated with it.

Bug: 116824030
Bug: 171549575
Test: atest InputReader_test
Change-Id: I3f78a122a3a8dce9c8dac3c88f3d51f29afa367c