Add documentation to VibratorManager on foreground/background behavior

Add information to javadocs explaining that apps in background can
only vibrate the device if the given AudioAttributes usage is one of
notification, ringtone or alarm usages.

Bug: 146313440
Test: N/A
Change-Id: Ib0b8401539157f0ce2768aab281f3b23aae38dae

core/java/android/os/VibratorManager.java

@@ -94,6 +94,8 @@ public abstract class VibratorManager {
      * VibrationEffect VibrationEffects} to be played on one or more vibrators.
      * </p>
      *
+     * <p>The app should be in foreground for the vibration to happen.</p>
+     *
      * @param effect a combination of effects to be performed by one or more vibrators.
      */
     @RequiresPermission(android.Manifest.permission.VIBRATE)
@@ -109,6 +111,9 @@ public abstract class VibratorManager {
      * VibrationEffect} to be played on one or more vibrators.
      * </p>
      *
+     * <p>The app should be in foreground for the vibration to happen. Background apps should
+     * specify a ringtone, notification or alarm usage in order to vibrate.</p>
+     *
      * @param effect a combination of effects to be performed by one or more vibrators.
      * @param attributes {@link VibrationAttributes} corresponding to the vibration. For example,
      *                   specify {@link VibrationAttributes#USAGE_ALARM} for alarm vibrations or