Fix RecyclerView crash in device discovery list

Resolves an IndexOutOfBoundsException when updating the list of
discovered companion devices.

The adapter was using a shared, mutable list reference, causing its
internal state to be out of sync.

The fix is to have the discovery service create a new list snapshot on
each update and for the adapter to correctly calculate diffs for
notifyItemRangeInserted().

Test: CTS
Fix: 425323417
Flag: EXEMPT bugfix
Change-Id: I90a445b163ac1c02d762ad273e5250b21167180e