With media client APIs migrated to retrofit (#2998)

* With media client APIs migrated to retrofit

* Add test cases and java docs

* Fix test

* Fix build

* Fix build and other minor issues

* Fix tests

app/src/main/java/fr/free/nrw/commons/media/MediaClient.java

@@ -0,0 +1,46 @@
+package fr.free.nrw.commons.media;
+
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+
+import io.reactivex.Single;
+
+/**
+ * Media Client to handle custom calls to Commons MediaWiki APIs
+ */
+@Singleton
+public class MediaClient {
+
+    private final MediaInterface mediaInterface;
+
+    @Inject
+    public MediaClient(MediaInterface mediaInterface) {
+        this.mediaInterface = mediaInterface;
+    }
+
+    /**
+     * Checks if a page exists on Commons
+     * The same method can be used to check for file or talk page
+     *
+     * @param title File:Test.jpg or Commons:Deletion_requests/File:Test1.jpeg
+     */
+    public Single<Boolean> checkPageExistsUsingTitle(String title) {
+        return mediaInterface.checkPageExistsUsingTitle(title)
+                .map(mwQueryResponse -> mwQueryResponse
+                        .query().firstPage().pageId() > 0)
+                .singleOrError();
+    }
+
+    /**
+     * Take the fileSha and returns whether a file with a matching SHA exists or not
+     *
+     * @param fileSha SHA of the file to be checked
+     */
+    public Single<Boolean> checkFileExistsUsingSha(String fileSha) {
+        return mediaInterface.checkFileExistsUsingSha(fileSha)
+                .map(mwQueryResponse -> mwQueryResponse
+                        .query().allImages().size() > 0)
+                .singleOrError();
+    }
+}

app/src/main/java/fr/free/nrw/commons/media/MediaInterface.java

@@ -0,0 +1,28 @@
+package fr.free.nrw.commons.media;
+
+import org.wikipedia.dataclient.mwapi.MwQueryResponse;
+
+import io.reactivex.Observable;
+import retrofit2.http.GET;
+import retrofit2.http.Query;
+
+/**
+ * Interface for interacting with Commons media related APIs
+ */
+public interface MediaInterface {
+    /**
+     * Checks if a page exists or not.
+     * @param title the title of the page to be checked
+     * @return
+     */
+    @GET("w/api.php?action=query&format=json&formatversion=2")
+    Observable<MwQueryResponse> checkPageExistsUsingTitle(@Query("titles") String title);
+
+    /**
+     * Check if file exists
+     * @param aisha1 the SHA of the media file to be checked
+     * @return
+     */
+    @GET("w/api.php?action=query&format=json&formatversion=2&list=allimages")
+    Observable<MwQueryResponse> checkFileExistsUsingSha(@Query("aisha1") String aisha1);
+}