Add OBSERVE_NETWORK_POLICY for shell to test CTS

Both of NetworkPolicyManager#isUidNetworkingBlocked() and
NetworkPolicyManager#isUidRestrictedOnMeteredNetworks() are
required OBSERVE_NETWORK_POLICY but OBSERVE_NETWORK_POLICY is a
signature permission that CTS cannot acquire.
So add this permission for shell and call these 2 APIs with
shell identity to fix the problem.

Bug: 176289731
Test: atest CtsHostsideNetworkTests:HostsideNetworkPolicyManagerTests
Change-Id: Iba7321c82e1a3ed773f99ccd4b9e3c24c607353f