Make low storage notification PendingIntent immutable

Malicious apps, especially NotificationListeners can hijack such
intents and grant themselves access to sensitive URIs by relying on
the OS URI granting mechanism. This change mitigates the security
issue and also adheres to the new enforcement requiring Android S+
targeting apps to explicitly specifiy a mutability flag.

Test: Manual
Bug: 153466381
Change-Id: If478386d68cf25c8f93c2685824700679e6b9ea9