SharedPreferences$Editor.startCommit()

Adds a fire-and-forget save method (startCommit) to the
SharedPreferences.Editor, which is the way most people use it anyway.

This commit adds the implementation.  The previous commit added the
interface and docs:

   previous change: Idf9934b445da1fb72b79f0192218b47c0a7f5a34
        git commit: edf32d01

In addition, this change:

-- adds a generic "runPendingWorkFinishers" mechanism to
   ActivityThread to wait on async operations that are still
   in flight and use it for this.

-- ties runPendingWorkFinishers into Activity.onPause,
   BroadcastReceiver, and Service.

-- makes sSharedPreferences keyed on name, not File, to avoid
   unnnecessary allocations

-- documents and guarantees what thread
   OnSharedPreferenceChangeListener callbacks run on

-- makes a few things in frameworks/base use startCommit(), notably
   Preference.java (which was ignoring the return value anyway)

Change-Id: I1c8db60ad45643226fe6d246d3e513eeb7bd0ebd