Moved some functions to DumpstateUtil.h.

dumpstate_board() is been refactored into a HIDL interface, and the HIDL
implementations will need help functions to dump files and run commands into
a file descriptor.

BUG: 31982882
Test: dumpstate_test passes
Test: manual verification

Change-Id: I7a32f0ac236dae34fd85abe47bed0e52a34c5f36