Bluetooth: AT: Improve API() to work with buffer increment

at_get_number which converts the string into number now operates on
at_client to get the string buffer. This patch also improves API()
skip_whitespace to be operated on at_client. Also the the API's
get_cmd_value, get_response_string are updated to work with buf
increment.

Also in this patch the return type of the function str_has_prefix
is changed from 'int' to 'bool'.

Change-Id: Ia626e0d13212b84413cce0444349975f4abe1cf6
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>