Allow the system to register 250 NetworkCallbacks.

Give anyone with PERMISSION_MAINLINE_NETWORK_STACK (i.e.,
either the system or the networkstack process) a separate limit
of 250 callbacks per UID.

Bug: 183921387
Test: new unit tests
Change-Id: I24580ea48e3ad502ef584efc5fde0b5d22e392b4
2 files changed