HDMI: Migrate CEC settings from Global to SharedPreferences

This change introduces a one-time migration mechanism for HDMI CEC settings. Before Android S CEC settings were stored in Settings.Global and are now stored in a dedicated SharedPreferences file.
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 power_control_mode "to_tv" // 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: I60db5f356916e8dd01170a91a6c8103705b6258c