Make TARE DeviceConfig-urable.

Make it possible to configure TARE factors via DeviceConfig. This
required standardizing the default value units. Values set locally (via
adb and/or developer options) will override DeviceConfig.

Also update the README with a few more details and clarifications.

Also adjust some of the default values based on fishfooding experience.

Bug: 158300259
Bug: 189850067
Bug: 205624100
Test: Change values via Developer Options UI
Test: Change values via adb
Change-Id: I71a90184bf2bf4d206a3802823f36bfafa8e3009