Bug fixes of DRM framework.

- Add death listener to clean-up drmserver appropriately
  when drmserver died.
- Remove "static" declaration of mUniqueIdVector because it was not
  needed to be static variable.
- Remove "class DrmContentIds;" because the class does not exist.
- contentPath in saveRights() could be empty because
  it is not required by some DRM schemes.
- Fix naming convention to use sXXX for static variables.
- Fix typo

Change-Id: I7d440488fc074c200f1009d1bafafeffebd690b2