GD-Shim: Refactor shim layer

* Fix a memmory leak in GetGabeldorscheStack() by allocating singleton
  as static variables instead of heap allocated variables
* Move stack.h/cc from GD to MAIN as such code is not required for GD's
  operation
* Create handler for Stack so that legacy stack callback can run on such
  handler
* Use GD alarm to do scan and observe timers in Btm
* Make Btm park of stack so that we don't have to keep a static copy of
  Btm in btm_api.cc

Bug: 160101414
Tag: #gd-refactor
Test: enable and disable GD via shim
Change-Id: I5521773ac70070d0402afde14707d0e4665a0b10