Fixup HttpPageLoader _loadPage (#8984)

Fixup for e4bc8990 (#8955)

HttpSource.fetchImage() uses Call.asObservableSuccess(), which
cancels the call on unsubscribe. This causes the call to be cancelled
before it is used, leading to a "java.net.SocketException: Socket is
closed" when trying to use the response in putImageToCache().

To fix this, use Call.awaitSuccess() via a new HttpSource.getImage()
suspending function. This addition to source-api is only intended for
app use, so it will not be added to the extensions-api stubs.

(cherry picked from commit b4b3a4d2869fae7839b4b3111e289056e33cfea8)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/ui/reader/loader/HttpPageLoader.kt

app/src/main/java/exh/util/DataSaver.kt

@@ -27,6 +27,16 @@ interface DataSaver {
                     page.imageUrl = imageUrl
                 }
         }
+
+        suspend fun HttpSource.getImage(page: Page, dataSaver: DataSaver): Response {
+            val imageUrl = page.imageUrl ?: return getImage(page)
+            page.imageUrl = dataSaver.compress(imageUrl)
+            return try {
+                getImage(page)
+            } finally {
+                page.imageUrl = imageUrl
+            }
+        }
     }
 }