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.swcodec.apex shows
...
X.509, CN=com.android.media.swcodec, OU=Android, O=Android, L=Mountain View, ST=California, C=US

Change-Id: I3a967fa640ce77177763b78a34a2df05f70ce60f

apex/Android.bp

@@ -53,6 +53,7 @@ apex {
     ],
     use_vendor: true,
     key: "com.android.media.swcodec.key",
+    certificate: ":com.android.media.swcodec.certificate",
 }
 
 apex_key {
@@ -71,3 +72,8 @@ android_app_certificate {
     name: "com.android.media.certificate",
     certificate: "com.android.media",
 }
+
+android_app_certificate {
+    name: "com.android.media.swcodec.certificate",
+    certificate: "com.android.media.swcodec",
+}

apex/com.android.media.swcodec.x509.pem

@@ -0,0 +1,34 @@
+-----BEGIN CERTIFICATE-----
+MIIF3jCCA8agAwIBAgIJAIM72JpD4v6XMA0GCSqGSIb3DQEBCwUAMIGCMQswCQYD
+VQQGEwJVUzETMBEGA1UECAwKQ2FsaWZvcm5pYTEWMBQGA1UEBwwNTW91bnRhaW4g
+VmlldzEQMA4GA1UECgwHQW5kcm9pZDEQMA4GA1UECwwHQW5kcm9pZDEiMCAGA1UE
+AwwZY29tLmFuZHJvaWQubWVkaWEuc3djb2RlYzAgFw0xOTAyMTEwMjExMTFaGA80
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+ccyPYgsdABtopbjN92rueu8sfF8R6ROy+tNgb6OjpAAevtnBfZ2LXqfObKirHCK+
+k6w4WSB1UUoZ3Xgz8sJtXgokvYeInkN8tHuTagHYU2VQTcA0rdBGMN/1OljJpWlN
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+nPXfgEJJ8AP1iRGibvew3i3SB6XTWFQYTUIBeJfDz/KDXQabP+yzXWISdZCUMUpx
+f+Raqsb5MoKaJdVgnSL0mBunjCyJDzzg34J7oGx6/BnwoiOrwLN4Qaz5U8jbrPSx
+p9LfleCcO7ZdeE8GKqx0X1T4d7tradtmxOS8Iwr4niskkHGRkzozvVvuyGKmoN2k
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+JTgH8P9mD1loYxRdo+vet8eQ
+-----END CERTIFICATE-----