Write file on boot when migration flag is enabled.

We are enabling a new flag storage via a server flag, but need to read the value of this flag early in the boot. We're achieving this by writing a file when a server-flag is enabled, in a location that's accessible early in the boot.

Bug: 328444881
Test: adb shell device_config put core_experiments_team_internal com.android.providers.settings.storage_test_mission_1 true && adb shell cat /metadata/aconfig/flags/storage_migration.log
Change-Id: I96eeb6b3cdf0ccf8c0e67f7896a69fbe49684abb