libdm: Make ExtractBlockDeviceName() return its result

From https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.md

:
"Prefer using return values over output parameters: they improve
readability, and often provide the same or better performance (see the C++
Primer)." Implement this advice for ExtractBlockDeviceName(). This patch
does not change any functionality.

Bug: 194450129
Test: mm libfs_mgr libdm_test
Merged-In: I6363781163eba08e6128507b403200f472f68a59
Change-Id: I7d340b33281ebccded0836cd0b5a293e941f4043
Ignore-AOSP-First: Already in AOSP.
Signed-off-by: Bart Van Assche <bvanassche@google.com>