Add merging of apexkeys and apkcerts metadata

Bug: 131437873
Test: Built system-only and vendor builds for merge. Compared
resulting apkcertx and apexkeys text files to that of a monolithic
build,
Test: Created colliding entries in both apexkeys and apkcerts text
files and ensure the script exited with an appropriate error message.
Test: Created unit tests to cover both non-colliding and colliding
entries
Change-Id: I6e42ce682ffa9059344e8cd63ba3a720c1f93452