bluetooth_packetgen: Move checksum_type_checker.h and

custom_type_checker.h

These files used to be located inside system/gd/packet/parser but
are actually only referenced from the generated packet files.
It is more appropriate to place them within the packet runtime
implementation.

Test: m root-canal
Change-Id: Ie4d85de947e594ca9a14db749c599dc01fbe054c