Take state snapshot in IMMS#attachNewInputLocked()

With this CL, we can finally start taking a state snapshot every time
IMMS#attachNewInputLocked() gets called, which allows us to include
the last N (currently set to 16) history of such snapshots in dumpsys
from InputMethodManagerService:

  adb shell dumpsys input_method

This is a huge milestone for us to start relying more on historical
debugging rather than logcat-based debugging.

Test: Run `adb shell dumpsys input_method` to make sure that
      "mStartInputHistory" entries are updated.
Bug: 35079353
Change-Id: Icb29c4ef0703553ead8654d3255a554de6d1a9b4