Fix Phone App crash due to binary data in HSP/HFP stream.

The BMW 2005 E46 sends binary data amongst the AT commands. Log below.

There were a couple of problems when this happened.
o Off by one error causing overflow of read buffer.
o No validation that input can be parsed by NewStringUTF()

This commit fixes the off-by-one, and validates that the input is ASCII.

I also increased the read buffer size to 256 to make it less likely we split
commands across buffers. We will lose the command when this happens.

2010-03-24 16:00:29.934185 > ACL data: handle 1 flags 0x02 dlen 17
    L2CAP(d): cid 0x0040 len 13 [psm 0]
      0000: 29 ef 13 41 54 2b 43 49  4e 44 3f 0d 6a           )ï.AT+CIND?.j
2010-03-24 16:00:29.940502 < ACL data: handle 1 flags 0x00 dlen 38
    0000: 22 00 40 00 2b ef 3d 0d  0a 2b 43 49 4e 44 3a 20  ".@.+ï=..+CIND:
    0010: 31 2c 30 2c 30 2c 30 2c  34 2c 30 2c 35 0d 0a 0d  1,0,0,0,4,0,5...
    0020: 0a 4f 4b 0d 0a b0                                 .OK..°
2010-03-24 16:00:29.949657 > HCI Event: Number of Completed Packets (0x13) plen 5
    handle 1 packets 1
2010-03-24 16:00:30.004284 > ACL data: handle 1 flags 0x02 dlen 9
    L2CAP(d): cid 0x0040 len 5 [psm 0]
      0000: 29 ff 01 01 76                                    )ÿ..v
2010-03-24 16:00:30.137218 > ACL data: handle 1 flags 0x02 dlen 144
    L2CAP(d): cid 0x0040 len 140 [psm 0]
      0000: 29 ef 0e 01 02 a9 03 58  02 a9 03 64 02 a9 03 98  )ï...©.X.©.d.©..
      0010: 02 a9 03 98 02 a9 03 98  02 a9 03 98 02 a9 03 98  .©...©...©...©..
      0020: 02 a9 03 98 02 a9 03 98  02 a9 03 98 02 a9 03 98  .©...©...©...©..
      0030: 02 a9 04 d8 02 a9 03 a4  02 a9 03 b0 02 a9 03 bc  .©.Ø.©.¤.©.°.©.¼
      0040: 02 a9 03 c8 02 a9 03 fc  02 a9 04 38 02 a9 04 44  .©.È.©.ü.©.8.©.D
      0050: 02 a9 04 50 02 a9 04 84  02 a9 04 90 02 a9 04 9c  .©.P.©...©...©..
      0060: 02 a9 04 a8 02 a9 04 b4  02 a9 04 c0 02 a9 04 d8  .©.¨.©.´.©.À.©.Ø
      0070: 02 a9 04 d8 02 a9 04 cc  02 a9 10 7c 02 a9 10 d8  .©.Ø.©.Ì.©.|.©.Ø
      0080: 02 a9 10 e4 02 a9 10 f4  02 a9 11 6a              .©.ä.©.ô.©.j

Change-Id: I7ccff70bc95a7945d8ff03527764fd8f4da04d24
Bug: 2539053