Cleanup of CTS Preconditions work

- removed unused constant from ScreenLockHelper class
- used getProperty method of ITestDevice, instead of executeShellCommand
- modified BuildCheckTest to reflect use of 'getProperty' method above
- placed target preparer for Wifi check above apk installer, so apk
isn't installed until precondition check has passed
- removed unused imports in net module's PreconditionsTest
- added Wifi precondition check to content, text, webkit & widget modules

bug:23939594
Change-Id: I125b9937b974d142c0d2343f584516bfc713787a

tests/cts/net/AndroidTest.xml

@@ -14,15 +14,13 @@
 -->
 <configuration description="Test module config for net apis">
     <include name="common-config" />
+    <target_preparer class="com.android.compatibility.common.tradefed.targetprep.ApkPreconditionCheck">
+        <option name="apk" value="CtsNetPreconditions.apk"/>
+        <option name="package" value="android.net.preconditions"/>
+    </target_preparer>
     <option name="apk-installer:test-file-name" value="CtsNetTestCases.apk" />
     <test class="com.android.tradefed.testtype.InstrumentationTest" >
         <option name="package" value="android.net.cts" />
         <option name="runner" value="android.support.test.runner.AndroidJUnitRunner" />
     </test>
-
-    <target_preparer class="com.android.compatibility.common.tradefed.targetprep.ApkPreconditionCheck">
-        <option name="apk" value="CtsNetPreconditions.apk"/>
-        <option name="package" value="android.net.preconditions"/>
-    </target_preparer>
-
 </configuration>

tests/cts/net/preconditions/src/android/net/preconditions/PreconditionsTest.java

@@ -15,9 +15,6 @@
  */
 package android.net.preconditions;
 
-import android.content.Context;
-import android.net.ConnectivityManager;
-import android.net.NetworkInfo;
 import android.test.AndroidTestCase;
 
 import com.android.compatibility.common.preconditions.WifiHelper;