Comment the freezer debounce default

Add a comment on the freezer debounce default pointing out that
lowering the default is risky without good testing.  The CTS test
accepts values down to 5s but that is not mentioned here.

Flag: EXEMPT comment-only
Bug: 409219690
Test: atest
 * CtsAppExitTestCases
Change-Id: I64874f8f7b587be270920dd8172b413d2453b984