libdm: Export ExtractBlockDeviceName()

Make this function available to libdm users. A caller outside libdm will
be added by a later patch.

Bug: 194450129
Test: mm libfs_mgr libdm_test
Merged-In: I3e3560f3cdef8978eac644d5b53cf3851209c0c2
Change-Id: Ic05cc84565952662178bb649ec97cad6f76dcf92
Ignore-AOSP-First: Already in AOSP.
Signed-off-by: Bart Van Assche <bvanassche@google.com>