Shared accounts and sharing of apps

API and preliminary implementation for sharing primary user accounts with a secondary user.
AbstractAccountAuthenticator has new methods to retrieve and apply a bundle of credentials
to clone an account from the primary to a restricted secondary user. The AccountManagerService
initiates the account clone when it starts up the user and detects that the user has
a shared account registered that hasn't been converted to a real account.

AccountManager also has new hidden APIs to add/remove/get shared accounts. There might be
further improvements to this API to make shared accounts hidden/visible to select apps.

AccountManagerService has a new table to store the shared account information.

Added ability in PackageManager to install and uninstall packages for a secondary user. This
is required when the primary user selects a few apps to share with a restricted user.

Remove shared accounts from secondary users when primary user removes the account.

Change-Id: I9378ed0d8c1cc66baf150a4bec0ede56f6f8b06b