Set baseConfigBuilder as parent for localhostBuilder

If a base-config is set in the NetworkSecurityConfig, the implicit
localhost config should inherit its properties. In particular, if any
certificate entries are declared, they should be considered.

Bug: 436512764
Test: atest AdServicesServiceCoreProtectedAudienceUnitTests
Flag: com.android.org.conscrypt.net.flags.network_security_config_localhost
Change-Id: Ibec4a3e3bc61fcddf0bdf9634d90372633dcf0a2