2.1.2 Sensor graph changes

[zafrani]

• https://trello.com/c/Hjsvc7gz/84-smooth-graphs-sensor-cards
• https://trello.com/c/AZXHD3Ux/79-smooth-graphs-sensor-detail-views

@jimmymlu you may want to use this as reference for iOS.

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This PR does the following:

1. Only draw 1/3 of the values returned from the server. In addition we check to see if a skipped point is equal to the max value in the graph. If so we draw that too so the user doesn't see it missing.
2. If a graphs max - min value is less than 5 we reduce the min by 5. If the min can't be reduced we increase the max by 5. If the max was originally 0 we just set it to 1 instead of increasing by 5.
3. Increased corner path radius for smoother looking graphs.

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The following are images of the new graphs.

[simonc312]

@zafrani so doing this will make it so that the graph will never touch the min value if min can be lowered or touch the max value if the max value can be increased?