Refine lsdump_paths.txt dependencies

This commit moves $(LSDUMP_PATHS) from the prerequisite of
lsdump_paths.txt to the prerequisite of findlsdumps. This change makes
it possible to dump the list of lsdump files without actually building
all lsdump files.

Bug: 124829578
Test: make out/target/product/generic/lsdump_paths.txt
      # does not build the actual lsdump files
Test: make findlsdumps  # builds lsdump_paths.txt and all lsdump files
Change-Id: I924f1c0c61391e958521c376a061651f2a301adb