passpoint-r2: set CA certificate for remediation server

It needs the APIs to set/get the CA certificate for remediation server.

Bug: 116346527
Test: ./frameworks/base/wifi/tests/runtests.sh
Test: tested with R2 AP for connection and verified that the CA
certificate is saved into keyStore.
Test: tested with R1 credential
Change-Id: I8568935234a6197e83f1b997f145b7e98dc56497
Signed-off-by: Ecco Park <eccopark@google.com>