* Revert "Make device discovery asynchronous (#10803)"
This reverts commit 972be9c8.

* Revert "Use Xcode instruments to list devices (#10801)"
This reverts commit 37bb5f13.

This is to resolve a failure that looks related to a bad install of Xcode 8.0
on our build bots and should be reinstated when the infra issue is diagnosed
and resolved.

Instruments worked well when this was originally landed, and on the
following commit, but started failing two commits after this originally
landed. Manual invocation of instruments on the build host currently
results in:

```
dyld: Library not loaded: @rpath/InstrumentsAnalysisCore.framework/Versions/A/InstrumentsAnalysisCore
  Referenced from: /Applications/Xcode8.0.app/Contents/Developer/usr/bin/instruments
  Reason: image not found
Abort trap: 6
```

It appears the /Applications/Xcode8.0.app/Contents/Applications
directory (which contains Instruments) is missing on the host.