Sign each APEX with different container certificate

Each APEX is signed with different certificate. The test certificate
(along with the private key) is com.android.<name>.x509.pem and
com.android.<name>.pk8. The files are in the same directory as the APEX
is defined and is referenced via android_app_certificate module named
com.android.<name>.certificate.

The test certificate could then be overridden via
PRODUCT_CERTIFICATE_OVERRIDES := <apex_module_name>:<new_cert_module_name>

Test: jarsigner -verify -verbose -certs out/target/product/blueline/system/apex/com.android.media.apex shows
...
X.509, CN=com.android.media, OU=Android, O=Android, L=Mountain View, ST=California, C=US

Change-Id: Ic61a7d2ca41254bda79ee5bdd3faf6d429a24e39
diff --git a/apex/Android.bp b/apex/Android.bp
index c077a77..6e0a908 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -42,6 +42,7 @@
         },
     },
     key: "com.android.media.key",
+    certificate: ":com.android.media.certificate",
 }
 
 apex {
@@ -65,3 +66,8 @@
     public_key: "com.android.media.swcodec.avbpubkey",
     private_key: "com.android.media.swcodec.pem",
 }
+
+android_app_certificate {
+    name: "com.android.media.certificate",
+    certificate: "com.android.media",
+}