Add new Secure setting for communal mode.

This new setting will store the list of packages (apps) which the user
has enabled for communal mode.

This list is expected to change rarely, therefore we opted to store it
in Settings.Secure for simplicity as opposed to a custom file.

Test: Locally on device
Bug: 198325535
Change-Id: I9b652b5920737308da7441d324f9ef8c4d60f996