Update SetFlagsRule instruction ravenwood document

ravenwood should use the SetFlagsRule.DefaultInitValueType.NULL_DEFAULT
in the constructor of SetFlagsRule. Since ravenwood tests are executed
on the host, SetFlagsRule should initialize the flag value to NULL,
instead of reading from the device.

Test: N/A
Bug: N/A
Change-Id: I75828fe92bf1a9ca2ea708a66a348f5f569cea7a