Bug 854413 - [MMS][SMS] Move all DOM element property initialization into init methods

- Moved initialization into init() methods
- Rename properties to match disambiguated element ids (sans prefix: threads|messages)
- Shift code lines to avoid lint complaints

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Fix impossible race condition: ThreadListUI elements _CANNOT_ be accessed before they are actually initialized.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>