PendingIntent: Use rethrowFromSystemServer

Fixes an issue where during a crash or shutdown of system_server apps
would start getting unexpected nulls as results of PendingIntent.get...(),
causing them to crash and getting flagged by AUPT.

Change-Id: Ieec289921f125da469f83813ca3f5920b5794a31
Fixes: 34932613
Test: adb shell stop; verify that there are no crashes in logcat.