Move the kptr_restrict setting from init.rc to init.cpp.

Also ensure that it uses the highest supported value, and
abort if the value is not above a minimum threshold.

Test: Tested against the curent kernel (maximum value of 2,
      set to 0 by the kernel initially) and against a
      modified kernel (maximum value of 4, set to 4 by the
      kernel initially)

Bug: 30368199
Change-Id: I608db577258b68b390ffe96f452e1f7c0bc9ad8a