Re-build reference search index so they can be used mid-edit

The search index was only being built on initialization, rather than at
the time the list was being shown. By moving it to be built on open
the list will always be fresh. Also, just after building the index, we can
enable/disable the select existing button based on the number of
results.

Bug: 51689
Bug: 51848
Bug: 52000
Change-Id: Ib38b1b227edc3d61dc49fa58999358e7adf11b08
Merge "Re-build reference search index so they can be used mid-edit"