Use a WeakReferences in DreamServiceWrapper and DreamActivityCallbacks.

The DreamServiceWrapper and DreamActivityCallbacks are  handed across
process during onBind. As a result, it should not reference the parent
class as this will lead to a memory leak. This changelist wraps the
parent in a WeakReference to resolve this.

Test: manual. monitored heap usage and verified no leak
Bug: 360088321
Flag: EXEMPT bugfix
Change-Id: I99bde447a8844b9d15c1908139b987d097e233d3