Allow RollbackManager to downgrade apks on user builds

- If running on a debug build, same behaviour is preserved.
- In case of a user build, INSTALL_ALLOW_DOWNGRADE flag is respected
  only if it's system_server who commits the session.

Test: atest RollbackTest on user build
Test: manually tried to downgrade apk on a user build, it failed.
Bug: 125657835
Change-Id: I469f6513831cc727fc7e62ce1ff4f6ef472ab5e9