Use system api for shell command handling

ConnectivityService should not use ShellCommand since it's @hide
which does not accessible in mainline modules. Replace it with
system api for mainline.

Bug: 170598012
Test: adb shell cmd connectivity airplane-mode enable
Test: adb shell cmd connectivity airplane-mode disable
Change-Id: I47e7a371c19eb18eac643fe9a2ab481bec5743c2