Introduce a new APK attribute: updatableSystem, default true.

Specified in the manifest.
If a preloaded APK is marked updatableSystem=false, any request for an update is rejected.
If an APK marked updatableSystem=false is being installed, regardless of the updatableSystem state of the version it's replacing, the install is rejected.
Disregard this restriction adb running as root.

!!This also marks Shell as such package!!!

Bug: 266131956
Test: atest PackageManagerTest
Test: atest PackageManagerServiceUnitTest
Change-Id: I86622e48a368bed260949b62c558c3e6ee0df931