Remove debuggable restriction on testing purpose method

CM#setTestLowTcpPollingTimerForKeepalive() is hidden method for
CTS. The debuggable restriction will prevent it from running on
actual CTS. The method is protected by permission with parameter
range constraint. It should be enough for incorrect usage from
normal apps. Remove it to allow test case to run on actual CTS.

Bug: 259000745
Test: CTS is the follow up commit
Change-Id: I3d4627c75bae7411c6a72799643d3b29bbae4f2f
diff --git a/service/src/com/android/server/ConnectivityService.java b/service/src/com/android/server/ConnectivityService.java
index 394292e..98d9e1c 100755
--- a/service/src/com/android/server/ConnectivityService.java
+++ b/service/src/com/android/server/ConnectivityService.java
@@ -5024,9 +5024,6 @@
     @Override
     public void setTestLowTcpPollingTimerForKeepalive(long timeMs) {
         enforceSettingsPermission();
-        if (!Build.isDebuggable()) {
-            throw new IllegalStateException("Is not supported in non-debuggable build");
-        }
 
         if (timeMs > System.currentTimeMillis() + MAX_TEST_LOW_TCP_POLLING_UNTIL_MS) {
             throw new IllegalArgumentException("Argument should not exceed "