Fix a use-of-uninitialized-value warning.

This warning was generated by the static analyzer:
warning: The left expression of the compound assignment is an
uninitialized value. The computed value will also be garbage
        *mDirtyMask |= animator->dirtyMask();

This can be fixed by initializing dirtyMask before using it in
AnimateFunctor.

Bug: None
Test: Static analyzer no longer complains about this.
Change-Id: I892749079cf93a012f1bed059a3f0c8bbaed9d7e