Frameworks: Clean up SystemProperties

Clean up SystemProperties.java. Add annotations.

Clean up SystemProperties.cpp. Refactor for proper C++11.
Make sure C-string key construction is properly reused. Use
android::base functionality for actual reading.

Fix the test script to refer to the right location. Add some
test coverage.

Test: m
Test: frameworks/base/core/tests/systemproperties/run_core_systemproperties_test.sh --rebuild
Change-Id: I490577370da985f600fb1117e3c818d3f68bad5f