Add Default Browser App support and relax Hosts validation for AppLinks

- add private PackageManager APIs for setting/getting the default
Browser App package name
- serialize / deserialize the default Browser App package name per User

Also relax the Hosts name validation for the AppLinls feature. Now we
just care if the IntentFilter is having an HTTP or HTTPS scheme.

Change-Id: I4436f66ac6beff57e14f7f3a2a00b0b582c03be9