HDMI: Migrate settings from R/S to U+ using legacy keys

This patch improves the existing migration mechanism by using the legacy
keys and converting the int values of the settings to the new ones.

Bug: 429952373
Bug: 432362341
Test: 1. adb shell rm -f /data/system/shared_prefs/hdmi_migration_prefs.xml && adb shell rm -f /data/system/shared_prefs/cec_config.xml
      2. adb shell settings put global hdmi_control_auto_device_off_enabled 0 // This can be replaced with other values.
      3. adb reboot
      4. Check for migration logs and files (adb shell cat /data/system/shared_prefs/hdmi_migration_prefs.xml, adb shell cat /data/system/shared_prefs/cec_config.xml)
Flag: EXEMPT migration implementation
Change-Id: I91214df47efdb026aecd4c35a8295744c577b10e