[UidMigration] fix update uninstallation with sharedUserMaxSdkVersion

When a system app is re-enabled after the update is uninstalled, when
the system app has shared uid, the current code doesn't support
directly reusing the disabled package setting. Instead, the preloaded
version is re-scanned and installed as a new app, which can bring
breaking behavior of changed UIDs when the manifest has
sharedUserMaxSdkVersion.

This change fixes the bug where registerExistingAppId fails when it
comes to shared uid, therefore directly reuses the disabled package
setting, consistent with the behavior for non-shared-uid system apps.

FLAG: EXEMPT BUGFIX
Test: manually with system-app-test.sh
BUG: 454062218



(cherry picked from commit 6b5ea2f7fbf50313d46e54e0d8f8c18c398e4869)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:4b32b0bd6876ecc25d60d95431763bf618699880
Merged-In: I417cec27697a210416027e862a5e5d207d268b82
Change-Id: I417cec27697a210416027e862a5e5d207d268b82