Remove hidden annotation usage from NetworkStack

NetworkStack uses hidden android.annotation.* annotations by pulling the
source files from frameworks/base. This should not be necessary, since
androidx annotations can be used instead.

Remove usage of such annotations and replace with androidx annotations.

Test: m NetworkStack NetworkStackApiStable
Test: builds, boots, WiFi working
Bug: 139105333
Change-Id: I24e147ac4f01feaf53424430b4825b316ec154fe