Minor changes on dumpstate refactoring:

- Moved more attributes into Dumpstate class.
- Created a GetPath() method that encapsulates bugreportDir_.
- Moved TakeScreenshot() into Dumpstate class.
- Uses DropRoot() on TakeScreenshot().

BUG: 26379932

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: I2d96460f9244d4e257a215e2fb1f00dfd466e059