Moar Dumpstate refactoring and unit testing:

- Moves UpdateProgress() into Dumpstate object.
- Tests RunCommand.AsRoot().
- Tests RunCommand.DropRoot().
- Test update progress.

BUG: 26379932
BUG: 31807540
Test:  mmm -j32 frameworks/native/cmds/dumpstate/ && adb push ${ANDROID_PRODUCT_OUT}/data/nativetest/dumpstate_test* /data/nativetest && adb shell /data/nativetest/dumpstate_test/dumpstate_test
Change-Id: I364d097487e090d201eb2e5f08fc794dce555510