binder: Add getCertificate / addTrustedPeerCerticate.

getCertificate returns the self-signed certificate
on this context.

addTrustedPeerCertificate adds a peer certificate
as trusted by this context.

Test: binderRpcTest
Bug: 195166979
Change-Id: I0e4fadd8e3391dc39f551e4b80e9411b16b696ab
3 files changed