Add enable and disable ip serving common functions

This is a op CL which prepare for refactor.

Bug: 185649441
Bug: 189410000
Bug: 178116595

Test: atest TetheringTests
Original-Change: https://android-review.googlesource.com/1730548
Merged-In: I270ff43c01736b9d18bd40a8e88e1379e9e4e863
Change-Id: I270ff43c01736b9d18bd40a8e88e1379e9e4e863
1 file changed