ASACORE-47 Don't reference qcc/StringMapKey.h in public header files

Referencing qcc/StringMapKey.h from alljoyn/ProxyBusObject.h will
break building any external users of ProxyBusObject since
qcc/StringMapKey.h is not publicly available.

This results in moving private static member variables from
ProxyBusObject.h to static, file-scope variables in ProxyBusObject.cc.

Change-Id: If7432456bc52f977312d0f184a0272cfc99cbae2
Signed-off-by: Steve Kinneberg <stevek@qce.qualcomm.com>