Start extensions in order of priority, start extensions that support multiple session modes first.

Currently, extensions with 'unlock-dialog' session modes are loaded randomly like all other extensions. If there is a regular 'user' session mode extension, loaded prior to the 'unlock-dialog' extension, when the session is locked, the shell in disabling the 'user' session mode extension has to first disable the 'unlock-dialog' extension, then disable the 'user' extension and then re-enable the 'unlock-dialog' extension again. After the first lock screen, the 'unlock-dialog' extensions are all ordered in the beginning and further lock screens work without disabling the 'unlock-dialog' again.

This defeats the purpose of an extension running in the 'unlock-dialog' session mode as it cannot run continuously.

This MR attempts to start extensions that support multiple session modes first. This should start the 'unlock-dialog', 'user' extensions first, followed by only 'user' extensions or hypothetically only 'lock-screen' extensions.