Adds a legacy flag to enable Launcher BR metrics.

This is necessary because the aconfig flag requires reboot to enable, however backup/restore happens on the first load so the flag will not be ready.

Flag: LEGACY ENABLE_LAUNCHER_BR_METRICS TEAMFOOD
Test: compilation / presubmit
Bug: 305984208
Change-Id: Ida90b1bd485fb037ebf3dde7ad17bf4e462661e6