This moves the default yearMonth logic into a common ReportingModule, rather than the coarse-scoped BackendModule, which may not want the default parameter extraction logic, as well as moving the 'yearMonth' parameter constant to the common package it's used in. This also provides a basis for future consolidation of the ReportingEmailUtils and BillingEmailUtils classes, which have modest overlap.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183130311
