Null check before accessing mRemovedInfo in PMS

When a system APK removes sharedUserId after an OTA, PMS will crash as
it attempts to access mRemovedInfo. mRemovedInfo will not be set during
boot time, as package broadcasts are not sent for system image changes.

Bug: 229017590
Test: manual
Change-Id: I162675e79bec103420aa26522f164b7f72d7c43f