Update stylus handwriting javadoc indicating usage of WeakReference in callback

Update stylus handwriting methods to reflect usage of WeakReference by
framework and app developer is encouraged to hold reference, if needed. The WekReference was originally added in [1].

[1]: I494c1b8fe5abf5f15e97884be5ef37783517ad40

Bug: 335731544
Test: build manually
Flag: EXEMPT javadoc update
Change-Id: Ia8ce5b6667017ac949ffd38b3551b71ab8c3025e