Add tests for default localhost NetworkSecurityConfig

Three apps are added:
  1. Without network_security_config.xml, to test that
     cleartextTrafficPermitted is enabled by default.
  2. With an empty network_security_config.xml, to test that
     cleartextTrafficPermitted is enabled by default.
  3. A network_security_config.xml with a custom CA key, to test that
     no default is applied, and a TLS connection remains possible.

The test keys and test certificates were generated using make_certs.go
from CtsNetSecConfigDownloadManagerTestCases.

Bug: 398997783
Flag: TEST_ONLY
Test: presubmit
Change-Id: I4204d8670ce89690a008b41ceb0c5eae3a25d531