Small improvements to code quality and readability

Add annotations to methods and parameters.
Replace Jetbrains' @NotNull with Androidx' @NonNull annotatiation.
Make class variables static if possible.
Use constants for some Strings.
Simplify if conditions.

app/src/main/java/com/google/android/material/appbar/FlingBehavior.java

@@ -63,6 +63,7 @@ public final class FlingBehavior extends AppBarLayout.Behavior {
         return consumed == dy;
     }
 
+    @Override
     public boolean onInterceptTouchEvent(@NonNull final CoordinatorLayout parent,
                                          @NonNull final AppBarLayout child,
                                          @NonNull final MotionEvent ev) {