Add checking permission at the setDataSaverModeEnabled()

Add calling enforceCallingOrSelfPermission() to check
for NETWORK_SETTINGS permission at the setDataSaverModeEnabled().
When an application calls for setDataSaverModeEnabled() passing a true
parameter, the access to Internet from mobile data became unavailable.

Test: Run test app to call setDataSaverModeEnabled() with a true parameter.
Test: Check the access to Internet from mobile data is available.
Change-Id: I14af89923e61d81355052704081edf121ef7d06d
Signed-off-by: Sehee Park <sehee32.park@samsung.com>