Refactor InputMapper creation and collection type

Move the creation of InputMappers from InputReader into InputDevice
and change the collection type from a vector of raw pointers to a
vector of unique_ptrs.

Add helper functions for iterating over the mappers data structure.

InputDevice::addMapper(...) is preserved for test cases, except
rather than taking a bare pointer to a mapper, it creates the mapper,
adds it's mapper vector, and returns a reference to this mapper. The
unit tests have been updated for this change.

Test: atest inputflinger_tests libinput_tests
Change-Id: I1e1b69e8bd13dcfa835b2f846fd5cf0d6a4e1719