Reduce frequency of file download progress updates (#6984)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1202552961248957/task/1211712368005490?focus=true

### Description

This PR updates file downloader to avoid emitting excessive number of
progress updates, which can have a negative impact on download
performance.

### Steps to test this PR

- [x] Smoke test downloading files, focusing on large downloads
(>100MB). See #6958 for some
examples.
- [x] Verify progress is displayed correctly in the system notification.

### No UI changes

Author: lmac012

downloads/downloads-impl/src/main/java/com/duckduckgo/downloads/impl/UrlFileDownloader.kt

@@ -117,24 +117,33 @@ class UrlFileDownloader @Inject constructor(
     ): Boolean {
         logcat { "Writing streaming response body to disk $fileName" }
 
-        // ensure content length never 0
-        val contentLength = if (body.contentLength() > 0) body.contentLength() else -1
+        val contentLength = body.contentLength().takeIf { it > 0 }
+        val calculateProgress: (Long) -> Int = if (contentLength != null) {
+            // Calculate real progress when content length is known
+            { bytesWritten -> (bytesWritten * 100 / contentLength).toInt() }
+        } else {
+            // Calculate fake progress when content length is not known
+            var progressSteps = 0.0
+            { floor(calculateFakeProgress(progressSteps) * 100.0).toInt().also { progressSteps += 0.0001 } }
+        }
+
         val file = directory.getOrCreate(fileName)
         val sink = file.sink()
         val source = body.source()
 
         var totalRead = 0L
         val buffer = Buffer()
         val success = try {
-            var progressSteps = 0.0
+            var progress = 0
             while (!source.exhausted()) {
                 val didRead = source.read(buffer, READ_SIZE_BYTES)
                 totalRead += didRead
                 sink.write(buffer, didRead)
-                val fakeProgress = floor(calculateFakeProgress(progressSteps) * 100.0).toInt().also { progressSteps += 0.0001 }
-                val calculatedProgress = (totalRead * 100 / contentLength)
-                val progress = if (calculatedProgress < 0L) fakeProgress else calculatedProgress
-                downloadCallback.onProgress(downloadId, fileName, progress.toInt())
+                val newProgress = calculateProgress(totalRead)
+                if (newProgress != progress) {
+                    progress = newProgress
+                    downloadCallback.onProgress(downloadId, fileName, progress)
+                }
             }
             true
         } catch (t: Throwable) {