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With crbug.com/981283 resolved, we can now fetch APK size data from the
newer timeseries2 API.

Also take the chance to do a bit of cleanup when building timeseries
keys from dicts.

TBR=benmason@chromium.org

Bug: 900218
Change-Id: I060921a01e380693e0633905aa2a3c7c43f73ecc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692408Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676376}