Create our own background thread for uidmap

BackgroundThread is a hiddenApi, so create our own background
thread as recommended by this doc:
go/aml-system-service-mainline-module-design

Bug: 144930182
Test: Ran CTS/GTS and adb shell cmd stats print-uid-map
Change-Id: Ic71190d281d64dd6d8839edcab55eb0a6e15b555
1 file changed