logd: security buffer only AID_SYSTEM reader

- limit AID_SYSTEM uid or gid to read security buffer messages
- adjust liblog tests to reflect the reality of this adjustment

To fully test all security buffer paths and modes

$ su 0,0,0 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*
$ su 1000,1000,1000 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*
$ su 2000,2000,2000 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*

ToDo: Integrate the above individually into the gTest Q/A testing

Bug: 26029733
Change-Id: Idcf5492db78fa6934ef6fb43f3ef861052675651