WifiServiceImpl: Rename mClientHandler and fix incorrect usages

mClientHandler in WifiServiceImpl has been mistakenly used in a
few instances to interact with ClientModeImpl.
Fixed these usages, and renamed mClientHandler to be more clear
about the fact that it should only be used for interactions with
external client code.

Bug: 119823519
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: I1f393716802b31342a7fc3b06599bee2baaaae81
3 files changed