Remove trailing newlines from abort message.

The tombstone will add a newline after the abort message, so remove
any trailing newlines before saving/printing.

Bug: 196414062

Test: Unit tests pass.
Test: Set system property debug.debuggerd.translate_proto_to_text to 0
test: and unit tests still pass.
Change-Id: I0d3dc215eb5d8be93d99e5b9d4f0a14b1d61396d