ONS: Make PendingIntent IMMUTABLE am: 9bc5a8c2ba am: d56c547aa6

Original change: https://android-review.googlesource.com/c/platform/packages/services/AlternativeNetworkAccess/+/1566040

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I9f58d6f6d4c5d4dbb6a96b81910e2ce9acf16b4e