Ensure adapter is not null in AutoCompleteTextView

A runnable is posted for AutoCompleteTextView's PopupDataSetObserver, but
there's a possibility that mAdapter would be set to null between when
the runnable is run and the time it is posted.

This fixes a monkey crash seen in build 29176.

Change-Id: Ie7be51db580d6e9775533890e5f6902409bec84d
MonkeyCrashID: 2071507494::1632:123:521