Closes#10257
We show a warning banner in the extensions list and also rely on the system
alert popup if someone attempts to install without the permission already
granted.
* Add private extension install method
Private extensions are put inside private data directory of the running app, so
this kind of extensions can only be used by the running app and not shared with
other apps.
One limitation of private extension is the lack of deeplink handlers (if there's
any) since the extension APK is not installed to the system.
When both kinds of extensions are installed with a same package name, shared
extension (the one installed to the system) will be used unless the version
codes are different. In that case the one with higher version code will be used.
* update
