Simplify adb LinePrinter newline handling.

We had mostly-working hacks before, but it's time to just modify LinePrinter
to suit our needs. If we tell LinePrinter what kind of output we're giving
it, it can manage things automatically.

This fixes the minor bug where we'd sometimes have a blank line after an
error message.

Change-Id: I07ff52437f2402de311e237dd1b2dd338d9b668a