Refactor to facilitate historical allocations

BannerChooser filter and get operations are explicit.

AllocationContext used to encapsulate banner selection criteria.

Deployment:
* RecordImpression logging must be corrected to parse for "uselang"

Change-Id: I2f3a0d89b30517294565231e60657bb25a456572
Merge "Refactor to facilitate historical allocations"