Document system activity intents that may not be implemented

All of these activity-start intents might be unimplemented on some
Android products.  Document this to make sure that developers are
aware of the need to safeguard against this.

Bug: 68300743
Bug: 62201251
Bug: 69587018
Fixes: 77282739
Test: atest CtsContentTestCases:.AvailableIntentsTest
Change-Id: Ia2346d03ccb7f2bdad2b84ba9efff72413fdc3c2
diff --git a/core/java/android/content/Intent.java b/core/java/android/content/Intent.java
index fe48975..ea08110 100644
--- a/core/java/android/content/Intent.java
+++ b/core/java/android/content/Intent.java
@@ -1420,6 +1420,9 @@
      * Activity Action: Start Voice Command.
      * <p>Input: Nothing.
      * <p>Output: Nothing.
+     * <p class="note">
+     * In some cases, a matching Activity may not exist, so ensure you
+     * safeguard against this.
      */
     @SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)
     public static final String ACTION_VOICE_COMMAND = "android.intent.action.VOICE_COMMAND";