Fix slow viewers initialization

- Fix "race condition" that could result in viewers being initialized twice,
  i.e. perform twice the heavy initialization pre-processing.
- Make sure viewers initialization is performed
  while the "Initializing UI" progress bar is displayed

Fix: 317477918
Test: npm run test:presubmit
Change-Id: I0bf83f36d21d3027632149b3ae121195fcaf9d59
1 file changed