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