Add loading notice while loading conversations

Summary

WIP: Since we are in string freeze and since this adds several strings, I guess it should not be merged until the 1.4 release

Add a loading indicator and message while loading conversations for the first time. If the phone is in doze mode, it often does not wake up for "awhile", so previously the page was blank and ugly for a long time.

Test Plan

• Leave phone unattended, screen off, for about 10 seconds
• Open SMS app (for the first time)
  • Before: Empty white screen with no indicator that it wasn't totally broken
  • Now: Loading indicator saying why it's taking so long
• Poke phone (literally. Any movement should get you out of doze, pushing the power button is probably not necessary) or wait "awhile"
• See that conversations load