Add system property for ime event log

Use a system property for DEBUG_IME_VISIBILITY
flag, so we can enable it for debugging.

Bug: 239501597
Test: Test as following steps:
      1. adb shell setprop persist.debug.imf_event 0
      2. adb shell am restart
      3. Observe logs
Change-Id: Ib419543f7f9b693a974aec7d520fff616bb2eafa