Show hidden API warning once per process

In order to not spam users with warning toasts, add a boolean flag
that guards the displaying of a warning message about hidden API
usage and is set after the first time a message is shown.

Bug: 64382372
Test: manual
Change-Id: If7ea995ddf4727a15eccf55dad42ef7775b1fc91

core/java/android/app/Activity.java

@@ -7071,7 +7071,10 @@ public class Activity extends ContextThemeWrapper
         boolean isApiWarningEnabled = SystemProperties.getInt("ro.art.hiddenapi.warning", 0) == 1;
 
         if (isAppDebuggable || isApiWarningEnabled) {
-            if (VMRuntime.getRuntime().hasUsedHiddenApi()) {
+            if (!mMainThread.mHiddenApiWarningShown && VMRuntime.getRuntime().hasUsedHiddenApi()) {
+                // Only show the warning once per process.
+                mMainThread.mHiddenApiWarningShown = true;
+
                 String appName = getApplicationInfo().loadLabel(getPackageManager())
                         .toString();
                 String warning = "Detected problems with API compatiblity\n"

core/java/android/app/ActivityThread.java

@@ -266,6 +266,7 @@ public final class ActivityThread {
     boolean mJitEnabled = false;
     boolean mSomeActivitiesChanged = false;
     boolean mUpdatingSystemConfig = false;
+    /* package */ boolean mHiddenApiWarningShown = false;
 
     // These can be accessed by multiple threads; mResourcesManager is the lock.
     // XXX For now we keep around information about all packages we have