Define new NetworkSecurityConfig for localhost

Some default settings for network security config such as
cleartextTrafficPermitted make no sense for localhost connections.

Define a default localhost NetworkSecurityConfig which is only applied
if the application does not already define a NetworkSecurityConfig for
localhost.

An hostname is considered to be localhost if either:
  - The hostname is "localhost" or "ip6-localhost"; or
  - InetAddress.isLoopbackAddress() returns true

Because of the size of the address space for what is considered
localhost, instead of adding entries to the existing Set returned by
getPerDomainConfigs(), a new method is added to ConfigSource:
getLocalhostConfig().

Bug: 398997783
Test: atest CtsNetSecConfigLocalhostCleartextTestCases
Test: atest CtsNetSecConfigLocalhostCleartextWithNscTestCases
Test: atest CtsNetSecConfigLocalhostTlsTestCases
Flag: com.android.org.conscrypt.net.flags.network_security_config_localhost
Change-Id: I608ff46ea5c976976a1b466f39be69cedea3a00c