Fix parser of AVRCP continuing messages

If packet_type is not START or SINGLE, we have to continue where we
stopped from previous packet. Therefore we must store where we left on
previous packet due to packet size limit. We store both the number of
attributes missing and the lenght of the last attribute that is missing.

An example interaction for this implementation, obtained with PTS test
TC_TG_MDI_BV_04_C (I reduced the MTU in order to reproduce it here and
values between brackets I added now):

> AVCTP: Command : pt 0x00 transaction 2 pid 0x110e
    AV/C: Status: address 0x48 opcode 0x00
      Subunit: Panel
      Opcode: Vendor Dependent
      Company ID: 0x001958
      AVRCP: GetElementAttributes: pt Single len 0x0009
        Identifier: 0x0 (PLAYING)
        AttributeCount: 0x00
< AVCTP: Response : pt 0x00 transaction 2 pid 0x110e
    AV/C: Stable: address 0x48 opcode 0x00
      Subunit: Panel
      Opcode: Vendor Dependent
      Company ID: 0x001958
      AVRCP: GetElementAttributes: pt Start len 0x0118
        AttributeCount: 0x04
        Attribute: 0x00000001 (Title)
        CharsetID: 0x006a (UTF-8)
        AttributeValueLength: 0x001b
        AttributeValue: isso eh um titulo mei longo
        Attribute: 0x00000003 (Album)
        CharsetID: 0x006a (UTF-8)
        AttributeValueLength: 0x00fe
	AttributeValue: super-long-album-name super-long-album-name
	super-long-album-name super-long-album-name super-long-album
	super-long-album-name [... snip... ] super-long-album-name-1234
> AVCTP: Command : pt 0x00 transaction 2 pid 0x110e
    AV/C: Control: address 0x48 opcode 0x00
      Subunit: Panel
      Opcode: Vendor Dependent
      Company ID: 0x001958
      AVRCP: RequestContinuingResponse: pt Single len 0x0001
< AVCTP: Response : pt 0x00 transaction 2 pid 0x110e
    AV/C: Stable: address 0x48 opcode 0x00
      Subunit: Panel
      Opcode: Vendor Dependent
      Company ID: 0x001958
      AVRCP: GetElementAttributes: pt End len 0x002a
        ContinuingAttributeValue: 678900000000000000
        Attribute: 0x00000005 (Track Total)
        CharsetID: 0x006a (UTF-8)
        AttributeValueLength: 0x0002
        AttributeValue: 30
        Attribute: 0x00000006 (Genre)
        CharsetID: 0x006a (UTF-8)
        AttributeValueLength: 0x0006
        AttributeValue: Gospel
1 file changed