Prevent race conditions after dropping database

Block all loaders from accessing the database
before indexing is complete.

Bug: 37501479
Test: make RunSettingsRoboTests
Change-Id: I2af98dcf6bceacbeffa70bd40879c97bb0cbc828
Merged-In: I02f8423c0ffc27abbb8ceb61a8c47d2f0796d0bb
9 files changed