Settings(wifi): Allow mainline wifi stack to use network request dialog

The wifi stack may not be running in system_server, so cannot get
NETWORK_SETTINGS permisison (platform signature). Guard it using the new
wifi mainline permission. If the wifi stack is running in system_server
(until it becomes a mainline module), it's uid (SYTEM_UID) should bypass all
permission checks.

Bug: 113174748
Test: Verified with CtsVerifier tests.
Change-Id: I87be97bb6e42c055d3aaa317a096256fd66f9e14