Follow-up: selectively enable elevation on working devices

[mcomella]

Vision statement / What / Requirements

A side effect of #305 was that the toolbar casted a shadow on the initial homescreen on all devices:

I realized we can check each device in code to selectively enable it on the known working devices.

Impact

Acceptance criteria

[mcomella]

QA: please verify the same steps as #305 (comment). Also, as depicted in the screenshot of the bug for this PR, on the initial homescreen there should be a shadow on the ES5 but not on the ES 1st gen or 2nd gen.

[nojunpark]

@mcomella Seems the opposite - there is a shadow on Echo 2 (the big screen one), but only a line dividing the toolbar and the bottom panel on ES 5 (small screen device)

[nojunpark]

Echo 5:

Echo 2:

[mcomella]

@npark-mozilla This looks like I'd expect.

[nojunpark]

Thanks!