debuggerd_client: increase pipe buffer size to max.

If a process tries to dump itself (e.g. system_server during ANRs),
crash_dump will block trying to write to its pipe if it's not
sufficiently large. Increase the pipe size to the max, and add a test
to make sure that it's always at least 1MB (the default value).

Bug: http://b/38427757
Test: debuggerd_test
Change-Id: Iddb0cb1e5ce9e687efa9e94c2748a1edfe09f119