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