Use ConnectivityCheckTargetPreparer in CTS

The preparer does basic checks for device configuration before any test
is run, so that wrong device configuration can be identified at the
module level, rather than fail individual test cases.

If test devices are not properly configured with a wifi configuration
and data-enabled SIM cards, the run will fail before starting the test
module.

As the preparer uses much of CtsNetUtils code for connecting to wifi,
refactor that code out of CtsNetUtils.

Test: atest CtsNetCasesLatestSdk
Change-Id: I1214b6d6916e836bcd68d15c50486092c7fb9a6e
diff --git a/tests/cts/net/Android.bp b/tests/cts/net/Android.bp
index 85942b0..1872327 100644
--- a/tests/cts/net/Android.bp
+++ b/tests/cts/net/Android.bp
@@ -60,6 +60,7 @@
     // uncomment when b/13249961 is fixed
     // sdk_version: "current",
     platform_apis: true,
+    required: ["ConnectivityChecker"],
 }
 
 // Networking CTS tests for development and release. These tests always target the platform SDK