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

src/com/android/launcher3/model/LoaderCursor.java

@@ -440,7 +440,8 @@ public class LoaderCursor extends CursorWrapper {
             if (item.screenId == Workspace.FIRST_SCREEN_ID) {
                 // Mark the first row as occupied (if the feature is enabled)
                 // in order to account for the QSB.
-                screen.markCells(0, 0, countX + 1, 1, FeatureFlags.QSB_ON_FIRST_SCREEN);
+                screen.markCells(0, 0, countX + 1, 1,
+                    FeatureFlags.getInstance(mContext).isQsbOnFirstScreenEnabled());
             }
             occupied.put(item.screenId, screen);
         }