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/org/schabi/newpipe/fragments/list/search/SearchFragment.java

@@ -724,7 +724,7 @@ public class SearchFragment extends BaseListFragment<SearchInfo, ListExtractor.I
     @Override
     public boolean onBackPressed() {
         if (suggestionsPanelVisible
-                && infoListAdapter.getItemsList().size() > 0
+                && !infoListAdapter.getItemsList().isEmpty()
                 && !isLoading.get()) {
             hideSuggestionsPanel();
             hideKeyboardSearch();