Add documentation about libcore benchmark tests

Also, add a hardcoded debuggable flag to Jetpack benchmark APK manifest.
It should be false by default, and recommend to hardcode it too.

Bug: 204160442
Test: n/a
Change-Id: I3ce73610b327857f18c5225f40492e147ec198a7
diff --git a/benchmarks/AndroidManifest.xml b/benchmarks/AndroidManifest.xml
index 8674b09..611b18b 100644
--- a/benchmarks/AndroidManifest.xml
+++ b/benchmarks/AndroidManifest.xml
@@ -17,6 +17,9 @@
 
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
           package="libcore.benchmark">
+    <!-- Important: disable debuggable for accurate performance results -->
+    <application android:debuggable="false"/>
+
     <instrumentation
         android:name="androidx.benchmark.junit4.AndroidBenchmarkRunner"
         android:targetPackage="libcore.benchmark"/>