There should be a way to clear additional subtypes.

InputMethodManager#setAdditionalInputMethodSubtypes() is the only API
that allows IMEs to add and remove additional subtypes.  However, due to
a bug, there is no way to clear the last entry of additional subtypes
because the API in question does nothing if the given array is emptry.

With this CL, an empty array is treated as a valid input.

This CL also adds a JavaDoc comment about a possible way to work around
this limitation in Android M and prior devices.

Bug: 26298984
Change-Id: I3731f84531247d071d9d88861e9079afc244a4e8