Output apkcerts file for android_app_set.

Soong and Make have no ways to figure out what splits will be outputted
from a given android_app_set, so it's impossible for them to provide
full PACKAGES.$(LOCAL_MODULE).CERTIFICATE entries, which are required to
build a final apkcerts.txt. This change makes extract_apks produce
apkcerts.txt files for each input modules instead. The Make-side
counterpart of this change merges all local apkcerts.txt into a final
one.

Bug: 160119159
Bug: 162464887
Test: main_test.go
Test: m apkcerts-list
Merged-In: I321e80fd636a955213761f56a3ac64bfe7f7f7c0
Change-Id: I321e80fd636a955213761f56a3ac64bfe7f7f7c0