Process: Make WIFI_UID a public API constant

The Process.WIFI_UID constant is referenced by the wifi mainline module & hence
needs to be a formal API constant. There are some other
subsystem uid's like PHONE_UID which are already public constants, so
there is precedence for making these uid's public if necessary?

Bug: 147396785
Test: Compiles
Change-Id: I85008965f9017e208c501c5a349f7e60cc102e0f