libutils: Fix an improper const-cast in RefBase

Under Fedora 15 Beta, gcc 4.6.0 warns:

  frameworks/base/libs/utils/RefBase.cpp: In member function
    ‘void android::RefBase::weakref_type::trackMe(bool, bool)’:
  frameworks/base/libs/utils/RefBase.cpp:483:67: error: passing
    ‘const android::RefBase::weakref_impl’ as ‘this’ argument of
    ‘void android::RefBase::weakref_impl::trackMe(bool, bool)’
    discards qualifiers [-fpermissive]

trackMe is not a const function, so don't use const in the static_cast
to a weakref_impl pointer.

Change-Id: I3c9ba73eb127985f5f54197ffecf2939c50f632c