Copy bootloader to PRODUCT_OUT

Implementation details:
- Import BOARD_PREBUILT_BOOTLOADER to Soong, and create a filegroup
  module
- Introduce `bootloader` property in android_device
- Introduce file copying (to PRODUCT_OUT) mechanism in android_device

Test: m aosp_cf_x86_64_phone_generated_device --soong-only
Bug: 388852084
Change-Id: I89a08590e39e771019e507b419fafae37bc4bb97