[adb incremental] send priority blocks first

Before this change, "Success" is returned after all data is streamed,
around 7.5 seconds for Megacity.

After this change, "Success" is returned in about 1.5 seconds, before
streaming finishes.

BUG: 151676293
Test: manual
Change-Id: Ifda7de48da8e82623c99ae0194f70cb162fd72fa