wifi_keystore: Convert CERT to PEM format

The public keystore API used by framework puts the cert in DER format.
So, convert the CERT to PEM format when supplicant retrieves the key to
maintain compatibility. Wifi was previously converting the cert to PEM format
and storing it in keystore using the privileged API surface.

Bug: 142089671
Test: Connected to passpoint networks
Test: Will send for full regression test
Change-Id: I78cf826287765c22dbbc5e198dd4655e0bf70a9f
2 files changed