Add "Call subject" capability to phone accounts.

Indicates whether the phone account supports displaying a subject line
for incoming calls and sending a subject line for outgoing calls.
Supports the IMS Instant Lettering feature.

Bug: 22769741
Change-Id: I6d803dc288bfd6d4e356310a8d10d5468c5d4186

telecomm/java/android/telecom/PhoneAccount.java

@@ -111,6 +111,15 @@ public final class PhoneAccount implements Parcelable {
     @SystemApi
     public static final int CAPABILITY_MULTI_USER = 0x20;
 
+    /**
+     * Flag indicating that this {@code PhoneAccount} supports a subject for Calls.  This means a
+     * caller is able to specify a short subject line for an outgoing call.  A capable receiving
+     * device displays the call subject on the incoming call screen.
+     * <p>
+     * See {@link #getCapabilities}
+     */
+    public static final int CAPABILITY_CALL_SUBJECT = 0x40;
+
     /**
      * URI scheme for telephone number URIs.
      */