Refactor boot_control into a separate library.

This extracts the implementation of boot_control into a new library,
libboot_control. The bootctrl.default module now wraps this library.
This refactoring will allow us to re-use the same implementation in
HIDL.

Bug: 138861550
Test: mm bootctrl.default
Change-Id: Ic0558da3c2d8e6f5ebec63a497825c24b51623b7