Lower the iterations on flutter_gallery__back_button_memory (#40102)

It was hitting the global devicelab 15 minute timeout.

dev/devicelab/bin/tasks/flutter_gallery__back_button_memory.dart

@@ -26,7 +26,7 @@ class BackButtonMemoryTest extends MemoryTest {
   Future<void> useMemory() async {
     await launchApp();
     await recordStart();
-    for (int iteration = 0; iteration < 10; iteration += 1) {
+    for (int iteration = 0; iteration < 8; iteration += 1) {
       print('back/forward iteration $iteration');
 
       // Push back button, wait for it to be seen by the Flutter app.

dev/devicelab/lib/framework/adb.dart

@@ -322,7 +322,7 @@ class AndroidDevice implements Device {
         process.exitCode.then<void>((int exitCode) {
           print('adb logcat process terminated with exit code $exitCode');
           if (!aborted) {
-            stream.addError(BuildFailedError('adb logcat failed with exit code $exitCode.'));
+            stream.addError(BuildFailedError('adb logcat failed with exit code $exitCode.\n'));
             processDone.complete();
           }
         });