Bluetooth: HFP HF: SLC query indicators present value

Service level connection sending AT+CIND? and parse the response
to get the present value of the available indicators.
This patch also providing callback interface to application for
the indicators value received.

Also added doxygen comment for connected and disconnected callback.

< ACL Data TX: Handle 256 flags 0x00 dlen 17                   [hci0] 25.251358
      Channel: 67 len 13 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x09 cr 0 dlci 0x02
         Control: 0xef poll/final 0
         Length: 9
         FCS: 0x40
        41 54 2b 43 49 4e 44 3f 0d 40                    AT+CIND?.@
> HCI Event: Number of Completed Packets (0x13) plen 5         [hci0] 25.262012
        Num handles: 1
        Handle: 256
        Count: 1
> ACL Data RX: Handle 256 flags 0x02 dlen 33                   [hci0] 25.293028
      Channel: 64 len 29 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x0b cr 1 dlci 0x02
         Control: 0xff poll/final 1
         Length: 24
         FCS: 0x86
         Credits: 1
        0d 0a 2b 43 49 4e 44 3a 20 30 2c 30 2c 31 2c 33  ..+CIND: 0,0,1,3
        2c 30 2c 33 2c 30 0d 0a 86                       ,0,3,0...
> ACL Data RX: Handle 256 flags 0x02 dlen 14                   [hci0] 25.295006
      Channel: 64 len 10 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x0b cr 1 dlci 0x02
         Control: 0xef poll/final 0
         Length: 6
         FCS: 0x9a
        0d 0a 4f 4b 0d 0a 9a                             ..OK...

Change-Id: I7b2a89487e2d481391c51201e32b3287808f82dc
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>