Check for errors in ThreadGroupReference JDWP commands

Returns INVALID_OBJECT error for null or invalid object. Also returns
INVALID_THREAD_GROUP error when the object is not a java.lang.ThreadGroup.

Removes unused Dbg::GetMainThreadGroupId method.

Bug: 17503230
(cherry picked from commit a06430c76981d545b5f2b64a7ef53c44c030cf73)

Change-Id: Ic39d3d2c45bf288fc22d908a3c90a3ca24f1c4d4
3 files changed