Update ImageStats test.

1) Add option to process more than one stats file name.

2) Metrics collected from files will be posted under the test
   named after the file name.

3) In addition to aggregate metrics, collect all the individual file
   sizes. Individual file sizes will be used at the time of debugging.

Parsing the json file will be taken care in the follow up CL.

Bug: b/203247205

Test: ImageStatsTest
Change-Id: Icd37cc3621d3e5e6e853628e36679a1c2e68c643
2 files changed