Clear uninstall block when removing DO.

Another way was to clear it using existing APIs for each package
but each call would cause Package Manager to re-serialize the
package-restrictions.xml, so I added a separate API to do it in
one go.

Bug: 149075700
Test: manual, set TestDPC as a DO, block uninstall, remove DO.
Test: manual, set TestDPC in COMP, block uninstall, migrate to COPE.
Change-Id: I9be69af5d7ae9e0ddda087d3e01e35f3429f25f4