Change KeyChain to assume PEM encoded keystore entries

Summary:
- Changed KeyChain to assume PEM encoded keystore entries
- Moved convertToPem from CertInstaller for reuse with other Credentials helpers
- Added convertFromPem for use decoding keystore entries

Change-Id: I340168b88aefa458d01e81324824e2e08b1d7c4e
2 files changed