Dialer: Move assertion to top of method

* Instead of using the number unchecked in various places and fail
  later when the value is null, move the assertion to the top of the
  method so we don't have to do all the work in between when we fail
  it anyway
* Also change the occurances of getPhoneNumber() so we don't have to
  call into that various times instead of once

Change-Id: Ibf7ab971dc1ee65e3d8bcc7668e9f9c8c3a30651
diff --git a/java/com/android/dialer/postcall/PostCall.java b/java/com/android/dialer/postcall/PostCall.java
index 542b8d0..376cb45 100644
--- a/java/com/android/dialer/postcall/PostCall.java
+++ b/java/com/android/dialer/postcall/PostCall.java
@@ -72,13 +72,14 @@
 
   private static void promptUserToSendMessage(Activity activity, View rootView) {
     LogUtil.i("PostCall.promptUserToSendMessage", "returned from call, showing post call SnackBar");
+    String number = Assert.isNotNull(getPhoneNumber(activity));
     String message = activity.getString(R.string.post_call_message);
     EnrichedCallManager manager = EnrichedCallComponent.get(activity).getEnrichedCallManager();
-    EnrichedCallCapabilities capabilities = manager.getCapabilities(getPhoneNumber(activity));
+    EnrichedCallCapabilities capabilities = manager.getCapabilities(number);
     LogUtil.i(
         "PostCall.promptUserToSendMessage",
         "number: %s, capabilities: %s",
-        LogUtil.sanitizePhoneNumber(getPhoneNumber(activity)),
+        LogUtil.sanitizePhoneNumber(number),
         capabilities);
 
     boolean isRcsPostCall = capabilities != null && capabilities.isPostCallCapable();
@@ -87,7 +88,6 @@
             ? activity.getString(R.string.post_call_add_message)
             : activity.getString(R.string.post_call_send_message);
 
-    String number = Assert.isNotNull(getPhoneNumber(activity));
     OnClickListener onClickListener =
         v -> {
           Logger.get(activity)