[Canvas] Provide information needed by View Switcher

Overrides getHousedViewManifestation in PanelInspector
such that it returns the currently housed view. This
is used by the view switcher (and potentially other
users); its absence causes nasa/mct#200.

 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Self-review (no other developer available)
 Describe potential impact on other areas of the code: Panel on canvas
 Describe any test prerequisites for this fix (e.g., restart the database): N/A
 For bugs marked "Regression - Yes", describe the cause of the regression: N
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N