Offload IMMS#mImeDrawsImeNavBarRes initialization

This is a follow up CL to my previous CL [1], which was needed to
propagate an overlayable resource config_imeDrawsImeNavBar value from
profile owner user to other profile users.

With this CL, the first initialization of

  InputMethodManagerService#mImeDrawsImeNavBarRes

will be moved from

  InputMethodManagerService#systemRunning()

to a background thread to minimize its impact on the device boot time.

Other than that, there should be no observable behavior difference.

 [1]: Id3d6a71d8ba1bfa49131350b68aa8d3424eca381
      f93769b1

Bug: 219820813
Fix: 223591305
Test: Manually verified as follows
  1. Build aosp_coral-userdebug then flash it.
  2. adb install -r TestDPC-normalv8001.apk
  3. adb shell am start -n com.afwsamples.testdpc/.SetupManagementLaunchActivity
  4. Set up work-profile
  5. adb shell am start --user 0 -n \
   com.android.documentsui/com.android.documentsui.files.FilesActivity
  6. adb shell dumpsys input_method | grep mNavigation
      -> "mNavigationBarController={mImeDrawsImeNavBar=false, ..."
  7. Enable gesture navigation
  8. adb shell dumpsys input_method | grep mNavigation
      -> "mNavigationBarController={mImeDrawsImeNavBar=true, ..."
  9. adb shell am start --user 10 -n \
   com.android.documentsui/com.android.documentsui.files.FilesActivity
 10. adb shell dumpsys input_method | grep mNavigation
      -> "mNavigationBarController={mImeDrawsImeNavBar=true, ..."
Change-Id: I20f6be5d676b9e61ea3077aeaa2eadc5dbbdcd0c