EthernetTetheringTest: toggle wifi while test network is not selected

Used to avoid flaky test because upstream changed events
order can't be guaranteed. Once tethering choose non-test
upstream {wifi, ..}, test network wont't be chosen.

Fix flaky tests which are using initTetheringTester():
testIcmpv6Echo
testTetherClatUdp
testTetherUdpV4_VerifyBpf
testTetherUdpV4
testTetherUdpV4Dns
testTetherUdpV6

Moreover, add permission ACCESS_WIFI_STATE to AndroidManifest.xml
to avoid nested runAsShell while using CtsNetUtils to access WIFI.

STACKTRACE:
java.lang.IllegalStateException: adoptShellPermissionIdentity calls must not be nested
at com.android.testutils.TestPermissionUtil.runAsShell(TestPermissionUtil.kt:49)
at com.android.testutils.TestPermissionUtil.runAsShell(TestPermissionUtil.kt:70)
at com.android.testutils.TestPermissionUtil.runAsShell$default(TestPermissionUtil.kt:65)
at com.android.testutils.TestPermissionUtil.runAsShell(Unknown Source:16)
at android.net.cts.util.CtsNetUtils.disconnectFromWifi(CtsNetUtils.java:293)
at android.net.cts.util.CtsNetUtils.disconnectFromWifi(CtsNetUtils.java:255)
at android.net.cts.util.CtsNetUtils.toggleWifi(CtsNetUtils.java:160)

Bug: 243314243
Test: atest EthernetTetheringTest
Change-Id: I814899ae71f5c302d490a0dd81d7210ac138faa5