Merge hidden API metadata into a single CSV file.

Add a python script to merge the CSV files, dealing with the fact that the
source columns may differ due to the annotation properties present in the
source.

Add this new file and the existing greylist.txt files as dependencies of
the 'dist' target, so that they appear as artifacts on the build server.
This allows them to be processed later by reading them from the build
server.

Test: m out/target/common/obj/PACKAGING/hiddenapi-greylist.csv
Bug: 117314178
Change-Id: Ib9e5da90e4c32333a3f4f02ee7f159f77086ae5e