changes the API setChannelMaxPowers from hide API to system API

This CL changes the hiden API `setChannelMaxPowers` to system API
to allow the privilege app to set the max power of each Thread channel.

Bug: b/346686506
Test: atest CtsThreadNetworkTestCases
Change-Id: I1089a4a315971c3bca8ec688a02143ddec93d039
9 files changed