Add the ability to restrict dreams to the system user.

By default, this functionality is disabled.
Which means that all users should continue to
be able to start dreams.

Bug: 213906883
Test: Manually with the following steps:
Step 0: Have 2 or more users on the device. Enable screen saver in
display settings. Optionally set the screen timeout to 15 seconds for
faster dreaming.
Step 1: With the system user (the first user created) as the
selected user, allow the device to fall into a dream. Make sure the
dream appears. Switch to a different user. Allow the device to fall into
a dream. Verify that the device behaves the same way it would without
this change (depending on other configuration settings, it may or may
not dream, but this change itself does not prevent any user from
dreaming).
Step 2: To verify that this change does prevent non-system users from
dreaming, set the config_dreamsOnlyEnabledForSystemUser config value to
true in core/res/res/values/config.xml and push the change to a device.
Switch to a non-system user and allow the device to fall into a dream.
The device should not display a dream (and instead, should just display
a black screen).

Change-Id: I2fa7a16ee7c6250a7a3ac49e82bb4c8085451391