monitor: Update path to hprof-conv

From tools r22.3, hprof-conv is inside platform-tools, not tools.
I've filed https://code.google.com/p/android/issues/detail?id=72570
to make a better fix in the future.

Change-Id: I21af21f9e2e6383c9d89be5d94c5474f42e32aec
1 file changed