Fix apex to override certificate

The rules for apex certificate:
1. <unspecified>: use <default app cerficicate>
2. name: use <default app cerficiate dir>/<name>(.x509.pem|.pk8)
3. :module: use specified by <module>

Certificates can be overridden by PRODUCT_CERTIFICATE_OVERRIDES.

Currently, 1) and 2) aren't overridden by PRODUCT_CERTIFICATE_OVERRIDES,
which should be.

Bug: n/a
Test: m (apex_test.go amended)
Change-Id: Icbdf4979613ef10127ecc02f3debd6a798460532