libcutils: move cutils/files.h to cutils/android_get_control_file.h

files.[h|cpp] is bound to be abused with junk, replace with
android_get_control_file.[h|cpp]. Plus some sundry cleanup.

Test: gTest libcutils-tests, logd-unit-tests, liblog-unit-tests,
      logcat-unit-tests and init_tests
Bug: 32450474
Change-Id: Ibd4a7aa4624ea19a43d1f98a3c71ac37805d36b5