Merge CSV files generated by UnsupportedAppUsageProcessor.

Flow:
1. Annotation processor generates a CSV file per class as a CLASS_OUTPUT resource.
2. hiddenapi.go extracts individual .csv files and merges them into an index.csv file per module.
3. hiddenapi_singleton.go merges individual index.csv files into a combined .csv file.

In a follow up hiddenapi-index.csv would replace unsupportedappusage_index.csv

Bug: 145132366
Change-Id: I87d92f9c8d4b1cc1df526fc576ee3c2101116b58
Test: diff unsupportedappusage_index.csv hiddenapi-index.csv