Fix perf tests for cold startup.

Kiling ia force-stopping was invalidating the entire roots cache.

Bug: 27743087
Change-Id: Ib788cfeaa6e60c40b7277ba7ca0f2371816eded1
diff --git a/app-perf-tests/AndroidManifest.xml b/app-perf-tests/AndroidManifest.xml
index 1c3ed80..0013b6b 100644
--- a/app-perf-tests/AndroidManifest.xml
+++ b/app-perf-tests/AndroidManifest.xml
@@ -2,6 +2,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.android.documentsui.appperftests">
 
+    <uses-permission android:name="android.permission.KILL_BACKGROUND_PROCESSES" />
+
     <application>
         <uses-library android:name="android.test.runner" />