liblogcat: replace NULL with nullptr

- Replace all NULL, macro defined to 0, with nullptr.  nullptr is a
  keyword of type nullptr_t, with a value of (nullptr_t)0, a pointer
  type of sizeof(void*) that can not confusingly promote to an int.
- Replace all boolean evaluations of values against 0, NULL, and
  nullptr with direct, ! or !! as appropriate.
- Note that thread_stopped should be semaphore, defer that to
  a non-code-quality improvement patch.
- Check for null context in android_logcat_destroy.
- Run clang-format to realign format with setting.

Test: compile and gTest logcat-unit-tests
Bug: 35326290
Change-Id: Iaf729cd7899c6cece78431536ed325604f0e353f