fix fx oracle decimals

[mendesfabio]

Description

Sync failed on Polygon due to null fxOracleDecimals -- this attribute is only set on Pool and Token creation (1y ago) but we're grafting based on a recent block. Since all the oracles FX tokens use (tokens are hard coded in the assets file) have 8 decimals, I'm changing the logic to set fxOracleDecimals on handleAnswerUpdated as well.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency changes
• Code refactor / cleanup
• Documentation or wording changes
• Other

Checklist:

• I have performed a self-review of my own code
• I have requested at least 1 review (If the PR is significant enough, use best judgement here)
• I have commented my code where relevant, particularly in hard-to-understand areas