Use getprop to get build properties.

The code used to read from builder properties from the file
"/system/build.prop". However in the recent change on the android side,
the critical prop "ro.build.product" is removed from this file which
causes issues like http://crbug.com/456431976#comment10

To be future-proof, this CL changes the code to get build properties
from output of `getprop`.

Bug: 476183863
Change-Id: I0ba0daf2838a6c0db616816594316de0e362ce98
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/7488129
Commit-Queue: Haiyang Pan <hypan@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>