[XPlat/ExposureNotification] RiskLevel/TransmissionRiskScore mappings are different between both OSs

[tmurakami]

Hi,

In Android, RiskLevel/TransmissionRiskScore mappings are:

transmissionRiskScores[0] when RISK_LEVEL_LOWEST (1)
transmissionRiskScores[1] when RISK_LEVEL_LOW (2)
transmissionRiskScores[2] when RISK_LEVEL_LOW_MEDIUM (3)
transmissionRiskScores[3] when RISK_LEVEL_MEDIUM (4)
transmissionRiskScores[4] when RISK_LEVEL_MEDIUM_HIGH (5)
transmissionRiskScores[5] when RISK_LEVEL_HIGH (6)
transmissionRiskScores[6] when RISK_LEVEL_VERY_HIGH (7)
transmissionRiskScores[7] when RISK_LEVEL_HIGHEST (8)

https://developers.google.com/android/reference/com/google/android/gms/nearby/exposurenotification/ExposureConfiguration#getTransmissionRiskScores()
https://github.com/google/exposure-notifications-internals/blob/7dadb83aa5c4b02ed4ae98cd8cf13f89b96962d2/exposurenotification/src/main/java/com/google/samples/exposurenotification/matching/RiskScoreCalculator.java#L69-L72
https://github.com/google/exposure-notifications-internals/blob/7dadb83aa5c4b02ed4ae98cd8cf13f89b96962d2/exposurenotification/src/main/java/com/google/samples/exposurenotification/matching/RiskScoreCalculator.java#L88-L91

On the other hand in iOS, they are:

transmissionRiskScores[0] for risk level 0
transmissionRiskScores[1] for risk level 1
transmissionRiskScores[2] for risk level 2
transmissionRiskScores[3] for risk level 3
transmissionRiskScores[4] for risk level 4
transmissionRiskScores[5] for risk level 5
transmissionRiskScores[6] for risk level 6
transmissionRiskScores[7] for risk level 7

https://developer.apple.com/documentation/exposurenotification/enexposureconfiguration/3586323-transmissionrisklevelvalues

For example, if TransmissionRiskScores is [1,2,3,4,5,6,7,8] and an exposure's risk level is 1, the resulting TransmissionRiskScore is 1 on Android and 2 on iOS, right?

How can we get the same value on both OSs?