Check if the end activity is exported for Dispatcher.

Poorly written applications can cause NFC stack to crash
by declaring intent filters such that their activity gets
resolved to receive Nfc Tag Intents, but explicitly mark
those activities to not be exported, causing SecurityException.

Check if the end activity is exported before starting the Activity.

Also adds a null pointer check for web links.

Bug: 79294832
Test: Manual; Test app to read tags and verifying no exceptions
Change-Id: Ife9f2ec10501c373105dc3866352fdc27456485a
(cherry picked from commit 7b0e76e1)