Bind SpellCheckerService as IMPORTANT_BACKGROUND

If both InputMethodService (IME) and SpellCheckerService (SCS) are
implemented in the same package and used at the same time, typing
something on an EditText can easily create an active spell checker
connection, which is likely to be kept alive even after the software
keyboard gets dismissed by the user.  Since currently
TextServicesManagerService (TSMS) binds SCS with
BIND_FOREGROUND_SERVICE_WHILE_AWAKE flag, the IME process continues to
be PROCESS_STATE_BOUND_FOREGROUND_SERVICE even after software keyboard
gets dismissed and InputMethodManagerService no longer binds IME with
BIND_FOREGROUND_SERVICE [1].  As a result, the IME process less
frequently receives UI_HIDDEN, with which HWUI heavily relies on to
purge its internal caches.

The idea of this CL is to reduce the process state bonus from TSMS to
SCS so that it can match with what IMMS does to IME when software
keyboard is not shown.  Hopefully this would not cause priority
inversion provlems between the focused application and SCS because
IMEs with hardware keyborad with no software keyboard have already
running on this process state.

This CL also makes sure that IME starts receiving UI_HIDDEN only
after it shows the software keyboard, like IME gains
BIND_TREAT_LIKE_ACTIVITY characteristics [2] only after it shows the
software keyboard.

 [1]: Id1f73de66dc93d63212183958a72119ad174318b
      2c84cfc0
 [2]: Ie5793fd9b40d980fa18f80246326511ed6ae0597
      f0f94d12

Fixes: 64378148
Test: Manually tested as follows
 1. make -j ApiDemos
 2. adb install -r $ANDROID_PRODUCT_OUT/data/app/ApiDemos/ApiDemos.apk
 3. tapas LatinIME
 4. adb install -r $ANDROID_PRODUCT_OUT/system/app/LatinIME/LatinIME.apk
 5. adb shell ime enable com.android.inputmethod.latin/.LatinIME
 6. adb shell ime set com.android.inputmethod.latin/.LatinIME
 7. adb shell "settings put secure selected_spell_checker 'com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService'"
 8. adb shell dumpsys activity processes com.android.inputmethod.latin
    -> make sure curProcState=7
    -> make sure there is no "pendingUiClean" entry.
 9. adb shell dumpsys activity services com.android.inputmethod.latin
    -> Make sure there is only one connection to "com.android.inputmethod.latin/.LatinIME"
      -> It should have following flags: "CR !FG IMPB !VIS"
10. Open "Api Demos" app
11. Go to Views/Text/EditText
    -> make sure software keyboard shows up
12. adb shell dumpsys activity processes com.android.inputmethod.latin
    -> make sure curProcState=3
    -> make sure pendingUiClean=true
13. adb shell dumpsys activity services com.android.inputmethod.latin
    -> Make sure there are two connections to "com.android.inputmethod.latin/.LatinIME"
      -> They should have following flags: "CR !FG IMPB !VIS" and "CR FGS LACT UI"
14. In the first EditText type "aaa" then hit the space key.
15. adb shell dumpsys activity processes com.android.inputmethod.latin
    -> make sure curProcState=3
    -> make sure pendingUiClean=true
16. adb shell dumpsys activity services com.android.inputmethod.latin
    -> Make sure there are two connections to "com.android.inputmethod.latin/.LatinIME"
      -> They should have following flags: "CR !FG IMPB !VIS" and "CR FGS LACT UI"
    -> Make sure there is a connection to "com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService"
      -> It should have following flags: "CR IMPB"
17. Tap the down button on the NavBar to dismiss the keyboard
18. adb shell dumpsys activity processes com.android.inputmethod.latin
    -> make sure curProcState=7
    -> make sure pendingUiClean=false
19. adb shell dumpsys activity services com.android.inputmethod.latin
    -> Make sure there is only one connection to "com.android.inputmethod.latin/.LatinIME"
      -> It should have following flags: "CR !FG IMPB !VIS"
    -> Make sure there is a connection to "com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService"
      -> It should have following flags: "CR IMPB"
Change-Id: I91bc58675ce2e257af11efce5572bd9ecd4a37ac