Added apis for changing the system whitelist

We want the user to be able to remove apps that are added in the system
whitelist. The user can restore them back later if they want.

Test: cts-tradefed run cts-dev -m CtsDeviceIdleHostTestCases

Bug: 63528174
Change-Id: Ia2f908c5fac84e28772d85b303be68b70efac8ba