Refresh visible IME proc boost upon onBindingDied

When an active IME is force-stopped, service connections from
InputMethodManagerService (IMMS) to that IME have died and
ServiceConnection#onServiceConnected() will never happen (again) even
if Context.BIND_AUTO_CREATE is specified hence IMMS needs to
re-establish connections from scratch.

Previously IMMS did not handle this scenario and
InputMethodManagerService#mVisibleConnection can remain to be true
even after the current IME is updated, force-stopped, or whatever
permanent death happened to it.  As a result, IMMS fails to give
process priority boost to the IME process when it shows up until it
becomes hidden then shown again.  Even though this is one-time
anomaly, not giving priority boost to a visible IME is not
intentional and should be fixed.

Luckily we can rely on a recently introduced callback
ServiceConnection#onBindingDied() [1][2] to clean up
InputMethodManagerService#mVisibleBound for such a scenario, and this
CL does it.

 [1]: I526cc00816c384fa9eb1312b92406f38085cbff9
      9484603c
 [2]: Ic21f7ef66d43fb6db9f7ebbc833f28c09474ea59
      ac265349

Fix: 80625728
Test: Manually verified as follows.
 1. Build and flash aosp_taimen-userdebug
 2. Run
      adb shell dumpsys input_method | grep mVisibleBound
    to make sure that mVisibleBound is true when and only when
    AOSP Keyboard is shown.
 3. Run
      adb shell dumpsys activity services com.android.inputmethod.latin
    to make sure that there are two connections to
    "com.android.inputmethod.latin/.LatinIME" when AOSP Keyboard is shown and
    is only one connection when AOSP Keyboard is hidden.
 4. Run
      adb shell dumpsys activity processes com.android.inputmethod.latin | grep curProcState
    to make sure that curProcState is
    PROCESS_STATE_BOUND_FOREGROUND_SERVICE (==4) when AOSP Keyboard is
    shown and curProcState is PROCESS_STATE_IMPORTANT_BACKGROUND (==6)
    when AOSP Keyboard is hidden.
 5. Run
      adb shell cat /proc/`adb shell pidof com.android.inputmethod.latin`/cgroup
    to make sure that cpuset is foreground when and only when
    AOSP Keyboard is shown.
 6. Run the following command while AOSP Keyboard is visible
      adb shell am force-stop com.android.inputmethod.latin
 7. Run
      adb shell dumpsys input_method | grep mVisibleBound
    to make sure that mVisibleBound is reset to false.
 8. Tap any EditText field
 9. Make sure mVisibleBound becomes true again
10. Make sure curProcState becomes
    PROCESS_STATE_BOUND_FOREGROUND_SERVICE again
11. Make sure cpuset for AOSP Keyboard is foreground
Change-Id: I983d95ffc0c2edab687117f6c6fa5a0d002ef6ad