Prefer WeakReference.refersTo() to get()

This is an initial pass over frameworks/core to avoid WeakReference
get(), since that can cause the referent to be unnecessarily retained.
Most of these were found by searching for "get() == null".

Bug: 215259836
Test: Treehugger
Change-Id: I5227b05ca22e83b886ccf6713989961076df3f8c