ANDROID: GKI: set CONFIG_STATIC_USERMODEHELPER_PATH

The android.security.cts.KernelConfigTest#testConfigDisableUsermodehelper
test is failing because CONFIG_STATIC_USERMODEHELPER_PATH is set to
an invalid value ("/sbin/usermode-helper") for the test. Setting to ""
instead.

Bug: 160949257
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I1d9f003c15a6b1d9a7f47804e50661a44a0725d5