BootReceiver: Skip subdirectories inside tombstones scan

Properly handle (vendor-specific) subdirs under tombstones
directory by skipping over them.

CRs-Fixed: 407529
Change-Id: I0cecd9fa629fad35d62d508ddaf13968c2e46f93
(cherry picked from commit 1440830e6ea3097278bc74762f09a21854d1b416)
(cherry picked from commit e042bd6f036a6dd3af4924b9a9111d9499259ae4)