Add explicit copy assignment operator for NotifyMotionArgs.

Right now we rely on an implicit definition of this operator, which is
deprecated since we have an explicit copy constructor. See
-Wdeprecated-copy-with-user-provided-copy for details on why this isn't
desirable. Fortunately, a default implementation of the copy assignment
operator is sufficient, so there's no actual bug in the current usages.

Test: atest NotifyArgs_Test.cpp
Change-Id: I58a913ad1c37414e86167557fdd4deb29499f432