Fix cleartext default for the manifest value

If no network security config is set the default value of
cleartextTrafficPermitted comes from the legacy
android:cleartextTrafficPermitted, default this to false for apps
targeting P to track the network security config behavior as was
intended.

Bug: 74587672
Test: atest CtsNetSecPolicyUsesCleartextTrafficUnspecifiedTestCases
Change-Id: I6367723befa1b947063554c9c061bdfc5be3d116