Make PendingIntent immutable

PacManager puts up a notification with PendingIntent, but this
PendingIntent is mutable that content can be changed by someone.
So make the PendingIntent to be immutable to fix the content.

Bug: 169790636
Test: atest FrameworksNetTests
Change-Id: I415c8ea715b8152c975a71bdd09822211fd89b5b