Improve D-Bus projection macros

Previously property maps need to be marked with attribute
dbus_propmap_field_enum and dbus_propmap_field_propmap since the macro
wasn't able to detect by itself. This patch improves the mechanism such
that the user does not have to declare the type but the macro can detect
by itself.

Bug: 186492781
Tag: #floss
Test: manual - build floss on Linux

Change-Id: Idf08d58824ac40e6bc6f02a1e6e3732674d8b12a