Add supportsDismissingSelfWindow attribute of IME

When a virtual keyboard is shown on some configurations (e.g. Phone),
the System UI may change the back navigation button to a different UI
element in order to dismiss the virtual keyboard. Such UI modification
is unnecessary when the virtual keyboard has a dismissing button on
its own window. This new attribute hints the System UI that the
virtual keyboard may have a UI element to dismiss itself. This will be
also useful for Tablet System UI which may not show a navigation bar
when a virtual keyboard is shown.

Bug: 34133139
Test: Add unit test InputMethodInfoTest
Change-Id: I0f6b130a7df57557e40b52a7b7ac00be965a17c3