API to set and clear country code

1.Added privileged API to set and clear
country code for testing.
2. Added privileged API to change the default country code.

Bug: 168743500
Test: atest android.net.wifi.cts.WifiManagerTest
Test: atest com.android.server.wifi
Change-Id: Idd09479d4f36bd5216b9a1e25c9d1ea815523e9d
10 files changed