display: Relocate low_battery_state() to reduce plugin duplication

display: Relocate low_battery_state() to reduce plugin duplication

Remove and redefine low_battery_state function in plugins/display/core.c

This function is added to below display-misc.
bool display_misc_is_low_battery_state(int val)
-> This function checks that val parameter is low battery status or not.

Actually, this function should be located under the battery module.
During refactoring, this function will be relocated.

Change-Id: I2c5d1ddb5df7bba56c4ac88ddcb92b8d0e3a9335
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>