Uses a system property to set dry-run mode.

On Android N, dumpstate was using a #define to set dry-run mode, which
allows dumpstate to run much faster, which in turn is useful when
developing new dumpstate features.

The drawback of this approach is that it requires modifying dumpstate.h
locally, building a new binary, and re-redeploying it in the device; a
better and more flexible approach is to use a system
property (dumpstate.dry_run) so developers can enable /disable it during
runtime.

Change-Id: I41d1e6ae3fa6ca2be23887f59d5ad4e9df7c10e6
Fixes: 30604041