Remove foreground power key code.

Remove code associated with foreground power key capture for system
apps, including the system API (reverts ag/30871623 and ag/30152916).

The decision was made to abandon this change for 25Q2 as Wallet does not
plan to use this feature with the system app limitation.

The flag is already rolled back in Gantry: see https://gantry.corp.google.com/changes/0a61p0a000001?build_unique_id=ComAndroidHardwareInputOverridePowerKeyBehaviorInFocusedWindowLaunch-platform&apply_default_filter=true&enable_workflow_timeline=false
Context: b/357144512#comment55

Test: atest PowerKeyGestureTests && atest GestureLauncherServicetest &&
atest WindowManagerServiceTests
Test: manually ensuring power button shortcuts remain same as before.
Bug: 357144512
Flag: com.android.hardware.input.override_power_key_behavior_in_focused_window

Change-Id: Ifd533b7e28745dd78be8ff53bd0cc465f248dda4