Installer (and uninstall) fallback fixes for API >= 24
Installer: Use content URI on API >= 24 for DefaultInstaller fallback
- When there's a permission mismatch (#951, #890), the fallback DefaultInstaller is invoked, which enforces file and content schemes for API < 24 and >= 24 respectively.
- Use content URI in that case, which allows the fallback to work.
PrivilegedInstaller: fallback to default installer for uninstall
- On API >= 24, in cases when the installer package name is not set to privext, the system won't let us uninstall.
- Fallback to the DefaultInstaller so that uninstall still works.