Correctly deal with dialogs in PackageInstallerActivity

- All UI (including dialogs) get recreated onCreate. Hence never restore
  any instance state
- Use suggested methods for dialog transactions.
- As all fragments get recreated after onCreate, no need to ever be
  afraid of fragment state loss.
- Persist mAllowUnknownSources when AnonymousSourceDialog was confirmed

Test: - Tried to install app and saw 'unknown sources' dialog. Rotated
        phone to go through activity lifecycle. Saw dialog recreated.
      - Confirmed 'unknown sources' dialog and rotated phone. Dialog was
        not shown again.
Fixes: 73119376

Change-Id: I58df73b806a11066e1117c2962e3219069a93431