input: use optionals for PropertyMap getters

Using optionals rather than returning a boolean forces the caller to
check whether the property is present, enforced by the compiler, and
also allows a more succinct syntax in some cases.

Bug: 245989146
Test: atest inputflinger_test
Test: connect Apple Magic Trackpad 2, check that gesture properties are
      set correctly in dumpsys input
Change-Id: Ia0fde1f67269e6e7149f297def626e572fd7790f