MediaMetrics: Cleanup the formatting of session ID

Use the flags Base64.NO_PADDING|Base64.NO_WRAP|Base64.URL_SAFE

This formats the id as one long string without == passing at the end,
Using -_ instead of +/ characters

Bug: 175357495
Test: atest android.media.metrics.cts.MediaMetricsAtomHostSideTests
Change-Id: Iba73976b5677e9a2e5fd3e156a55499005e26904

services/core/java/com/android/server/media/metrics/MediaMetricsManagerService.java

@@ -98,7 +98,8 @@ public final class MediaMetricsManagerService extends SystemService {
         private String getSessionIdInternal(int userId) {
             byte[] byteId = new byte[16]; // 128 bits
             mSecureRandom.nextBytes(byteId);
-            String id = Base64.encodeToString(byteId, Base64.DEFAULT);
+            String id = Base64.encodeToString(
+                    byteId, Base64.NO_PADDING | Base64.NO_WRAP | Base64.URL_SAFE);
             return id;
         }