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.

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

@@ -200,7 +200,7 @@ class HttpPageLoader(
 
             if (!chapterCache.isImageInCache(imageUrl)) {
                 page.status = Page.State.DOWNLOAD_IMAGE
-                val imageResponse = source.fetchImage(page).awaitSingle()
+                val imageResponse = source.getImage(page)
                 chapterCache.putImageToCache(imageUrl, imageResponse)
             }