Update ActivityManagerPerfTests README with atest

atest command now works

Test: atest -v ActivityManagerPerfTests
Change-Id: I4368199bacb2359d2ddef66ba9783ce1b663e2ab
diff --git a/tests/ActivityManagerPerfTests/README.txt b/tests/ActivityManagerPerfTests/README.txt
index 1040ed1..2686290 100644
--- a/tests/ActivityManagerPerfTests/README.txt
+++ b/tests/ActivityManagerPerfTests/README.txt
@@ -5,13 +5,7 @@
 * Self-contained perf tests should go in frameworks/base/apct-tests/perftests
 
 Command to run tests
-* atest .../frameworks/base/tests/ActivityManagerPerfTests/tests/
-  * Command currently not working: b/71859981
-* m ActivityManagerPerfTests ActivityManagerPerfTestsTestApp && \
-  adb install "$OUT"/data/app/ActivityManagerPerfTests/ActivityManagerPerfTests.apk && \
-  adb install "$OUT"/data/app/ActivityManagerPerfTestsTestApp/ActivityManagerPerfTestsTestApp.apk && \
-  adb shell am instrument -w \
-  com.android.frameworks.perftests.amtests/android.support.test.runner.AndroidJUnitRunner
+* atest -v ActivityManagerPerfTests
 
 Overview
 * The numbers we are trying to measure are end-to-end numbers