Add createAxis to reduce the size of a function

While refactoring input code to enable -Wshadow, I encountered the block
of code inside JoystickInputMapper that is used to create an Axis
object. This creation can be moved to a separate function for
simplicity.

Bug: 142017994
Test: atest inputflinger_tests
Change-Id: I76088ab054dcf24585837016fb57d6f2500ca5e3