AgileViewModel: update keyword for latest fixed tariff lookup

[ryanw-mobile]

Octopus' latest fixed tariff is 15M instead of 12M, so our existing keyword failed. Now we take OE-FIX- tariff prefix without specifying 12M

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:x: -2.00% (target: -1.00%)	:white_check_mark: 100.00%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (092d86861becab5d6e1dcb722c598c9dd04225eb) | 2640 | 2241 | 84.89% | | | Head commit (437ea999e3b2857b20f912c29365d6f777c7663b) | 2886 (+246) | 2392 (+151) | 82.88% (**-2.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#331) | 2 | 2 | **100.00%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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