Groundwork for runtime-toggleable feature flags

This is the first step in adding a flag toggler UI to launcher.
The change migrates a single flag (QSB_ON_FIRST_SCREEN) from a
boolean constant to a boolean method. In future, that will allow
us to return different values at runtime.

Bug: 117223984
Change-Id: I1e62c91dd941b8145166021bc0aa157733e62ea0