Bug 450368 - Embedding strings could be better encapsulated

Only nsEmbedString should need to call the various XPCOM nsEmbedString_*
functions. Other classes should work with nsEmbedString instances.

Change-Id: Ic83fe3335eb761b8d747c5d7fd8c5636ff663838
Signed-off-by: Neil Rashbrook <neil@parkwaycc.co.uk>
Signed-off-by: Lakshmi Shanmugam <lshanmug@in.ibm.com>