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.

Fixes: 160119159
Test: main_test.go
Test: m apkcerts-list
Change-Id: I321e80fd636a955213761f56a3ac64bfe7f7f7c0

java/androidmk.go

@@ -716,6 +716,7 @@ func (apkSet *AndroidAppSet) AndroidMkEntries() []android.AndroidMkEntries {
 				func(entries *android.AndroidMkEntries) {
 					entries.SetBoolIfTrue("LOCAL_PRIVILEGED_MODULE", apkSet.Privileged())
 					entries.SetString("LOCAL_APK_SET_MASTER_FILE", apkSet.masterFile)
+					entries.SetPath("LOCAL_APKCERTS_FILE", apkSet.apkcertsFile)
 					entries.AddStrings("LOCAL_OVERRIDES_PACKAGES", apkSet.properties.Overrides...)
 				},
 			},