libfiemap: Add a way to get the block device path of a mapped image.

This is needed for adb remount, to find the scratch device if it is
already mapped. Note that on devices without metadata encryption, this
cannot be done by querying device-mapper, since scratch will be a loop
device.

Bug: 134949511
Test: fiemap_image_test
Change-Id: Ia25d279c6f8a4838be32a8c01aefc67b5ec1e002