keystore: Fix PEM certs handling

Remove the DER encoding processing since keystore is guaranteed
to provide PEM encoded certificates.
Clean up hard-coded strings/values, and use better variable names.

Bug: 190223327
Test: Connect to enterprise networks using EAP-TTLS and EAP-TLS
to verify that both Root CA and client certificates are parsed
correctly, and verify logs.
Test: Regression test passed (b/195536535)

Change-Id: I79ca980599fa753392b5192e9d5872d435c815b0
1 file changed