halapi: Split the internal functions finely to get flexibility

halapi: Split the internal functions finely to get flexibility

New functions will use only __open_backend() without __close_backend()
in order to get the data of hal_backend structure. But this code have
been tightly coupled implemented in __get_backend and __put_backend.
In order to make the code more flexibility, split __get_backend/__put_backend
finley to two steps such as open/close and init/exit.

[Simple description for functions]
- __open_backend() : Open backend library and get backend symbol
- __close_backend() : Close backend library
- __init_backend() : After getting backend, initialize HAL backend driver
- __exit_backend() : After getting backend, exit HAL backend driver
- __get_backend()
	__open_backend()
	__init_backend()
	Increase usage_count
- __put_backend()
	__exit_backen()
	__close_backend()
	Decrease usage_count

[Change variable name in struct __hal_module_info]
- library_backend -> backend
- library_handle -> handle

Change-Id: I358e3f4db1991d26c8cf0498b3f37ec543da3e38
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>