lib: resource-monitor: Fix get_value_string issue by allocating string buffer

lib: resource-monitor: Fix get_value_string issue by allocating string buffer

The resource-monitor has the following policy of getting sting value
1. resource-monitor allocate the meory for string value
2. user of resource-monitor have to free the allocated memory

But, pass_resource_monitor_get_value_string didn't allocate
the memory for string value until now. Allocate memory for getting
string value on pass_resource_monitor_get_value_string.

Change-Id: I1517ab4362d754cb75b54d1887c1d031de14787d
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>