Create viewports in a separate function

This will allow the caller to re-use the viewport object for other
purposes.

That said, the whole thing is poorly designed, and there are many places
where we are mixing viewports and display ids, and not just in tests,
but also in production. For example, InputDevice has the concept of
associating with displays, but mappers also have a way to keep track of
their own associations.

The current approach allows us to slightly simplify some of the current
and new tests written in InputReader area.

Bug: 378308551
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Flag: TEST_ONLY
Change-Id: Ic9b476f3e991d128e429afcc096c9dc12e68f01e