dumpstate: enforce oneshot

Ensure the service exits after errors as well as after successful
finish as a oneshot service should.

While at it also move remove(file) to unlink(file) and fix an error
in checking the return value of unlink.

BUG: 123571915
Test: adb shell /data/nativetest64/dumpstate_smoke_test/dumpstate_smoke_test --gtest_filter=DumpstateBinderTest.*
Change-Id: I772b7981cd3b2f7c285ab980495d5539d57ebf46