Android: Remove ~50% of Java stream() usage.

Usage is against style guide, and this CL shows size savings from
avoiding them (and arguable more readable in many cases).

Bug: 344943957
Change-Id: I101d38afd3e1fb0d981ab5cb4d8032de4b72926a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5958767
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1375427}
NOKEYCHECK=True
GitOrigin-RevId: 62574d3dbe1a961a4fcb1be6f50667daa62a8f08

Author: agrieve

android/java/src/org/chromium/base/CollectionUtil.java

@@ -91,4 +91,13 @@ public static <T> List<T> strengthen(Collection<WeakReference<T>> weakRefs) {
         }
         return ret;
     }
+
+    /** Flattens a collection of collections. */
+    public static <T> List<T> flatten(Collection<? extends Collection<T>> input) {
+        List<T> ret = new ArrayList<>();
+        for (Collection<T> inner : input) {
+            ret.addAll(inner);
+        }
+        return ret;
+    }
 }