Remove utils::DeviceForMountPoint().

We only use it to get misc block device path, it can be replaced
with get_bootloader_message_blk_device(), and for writing bootloader
message, we can use update_bootloader_message() instead.

Also removed dependency on libfs_mgr since we are now using
libbootloader_message instead of using libfs_mgr directly.

Bug: None
Test: update_engine_unittests
Test: perform an OTA with powerwash
Change-Id: I80d0eea8e3ef6084820298cc0079da26fe1bfc63