Sync search result loaders

The loaders should be syncronized so the results can be
properly ranked without sudden insertions.

This means InstalledAppsLoader needs to finish faster,
which is accomplished by delaying icon loading to bind time
rather than as the apps are queried.

Bug: 34772522
Test: make RunSettingsRoboTests
Change-Id: I7f5244c574d37c6cfd8bbd0d3d40488f38211be3