Support metered Wi-Fi NetworkPolicy.

Add networkId field to NetworkIdentity to identify Wi-Fi networks by
SSID.  Add support for policies without usage cycles.

Only apply mobile policies when SIM state is ready, which is cleaner
than just checking for airplane mode.  Also avoids creating no-op
default policies when subscriberId is null.

Bug: 3001465, 3291052
Change-Id: I1f8aaa49a5db306df022c402ea7f3f5d4bc0cfc7