Use unique_ptr when creating InputChannel

InputChannel api will return unique_ptr, and it is up to the caller to
decide how to use that.

In most cases, the caller will proceed with making is a shared_ptr.

Bug: 142581626
Test: atest libinput_tests inputflinger_tests
Change-Id: Ice473a4c85241c63e3f4d6cdd30f8258ed7485e7