SF: Fix VSYNC injection

Disabling VSYNC injection causes the main thread to destroy the injected
EventThreadConnection, while it might be accessed by a Binder thread via
MessageQueue::invalidate.

Also, fix injection itself:

    - The EventThread did not dispatch injected events, falling back to
      fake VSYNC after timeout.
    - The MessageQueue would try listening to invalid FDs when enabling
      injection more than once.
    - Injection was not disabled after each test.

Finally, rename MessageQueue members only used for injection since S.

Bug: 150226265
Test: Toggle VSYNC injection in a tight loop while using phone.
Test: sffakehwc_test --gtest_repeat=-1 --gtest_break_on_failure
                     --gtest_filter='*VsyncInjection'
Change-Id: Ia58859cd8a36749bf56bb94fa724efe7c1b27b46