roomservice: Use ElementTree.write()

write() handles XML conversion and file writing internally.
The xml_declaration=True parameter adds the XML header
automatically, eliminating the need for manual string manipulation

Change-Id: Ib8c3fe33e60accad05572c5af0f0b90866a219d9
Signed-off-by: Jyotiraditya Panda <jyotiraditya@aospa.co>