Check security context in CA certificate injection

For binder clients trying to inject CA certificates, DNS resolver not
only checks its uid, but also security context. The purpose of these is
to reduce the possibility of certificate injection being misused except
for tests.

Bug: 162055050
Test: atest
      resolv_integration_test:ResolverTest#PermissionCheckOnCertificateInjection
Change-Id: I75bcf852d147b72c9045ec15ed3cb9684579fe3c