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

java/app_test.go

@@ -153,6 +153,9 @@ func TestAndroidAppSet(t *testing.T) {
 	if s := params.Args["allow-prereleased"]; s != "true" {
 		t.Errorf("wrong allow-prereleased value: '%s', expected 'true'", s)
 	}
+	if s := params.Args["partition"]; s != "system" {
+		t.Errorf("wrong partition value: '%s', expected 'system'", s)
+	}
 }
 
 func TestAndroidAppSet_Variants(t *testing.T) {