build: Fix ptr <-> integer cast warnings

This CL removes the -Wno-pointer-to-integer-cast and
-Wno-integer-to-pointer-cast flags from GN build files. The resulting errors
were fixed using the following:

  1. All ptr <-> integer casts are now done by using the new
     PTR_TO_INT/INT_TO_PTR macros defined in osi.h

  2. The TIMER_PARAM_TYPE macro, defined in gki/common/gki.h and
     include/bt_target.h have been redefined as void* rather than UINT32. This
     is better, since "void*" can act as a simple container without any
     precision loss that would be caused by a type such as UINT32 on 64-bit
     systems. void* inherently is a safer container for all pointer types but
     UINT32 isn't.

BUG=21570302

Change-Id: I4a82c4a40c91caa31e372382c40d424be220cbe3