Let Settings app use InputMethodInfo#loadIcon()

This is a follow up CL to my previous CL [1], which attempted to show
the most appropriate IME icon even when multiple IMEs are implemented
in a single APK.

This CL simplifies the way how IME icon is obtained in the Settings
app, by simply using InputMethodInfo#loadIcon(), which already does
what we want.

Notable user-visible behavior changes are:
 - Like before my previous CL was submitted, android:logo will not be
   used in the IME settings (again).
 - Icons shown in IME settings start having (user) badges like other
   settings pages.
 - If no icon is available in IME,
   PackageManager#getDefaultActivityIcon() will be used. This is
   actually consistent with other settings pages.

 [1]: I406ccc0d53e6ec69793c2fc8be8c6c1c90b34811
      2cae5b89

Bug: 28204635
Fix: 131432102
Test: Manually verified as follows.
  1. Build aosp_taimen-userdebug and flash it
  2. Install TestDPC.
  3. Set up a work profile
  4. Open Settings App -> System -> Languages & input
     -> Advanced -> Virtual keyboard for work
  5. Make sure the icon of "Android Keyboard (AOSP)" has a small badge.
  6. Tap "Manage keyboards"
  7. Make sure the icon of "Android Keyboard (AOSP)" has the same icon
     as the step 5.
  8. Open Settings App -> System -> Languages & input
     -> Virtual keyboard
  9. Make sure the icon of "Android Keyboard (AOSP)" does not have a
     small badge on it.
 10. Tap "Manage keyboards"
 11. Make sure the icon of "Android Keyboard (AOSP)" has the same icon
     as the step 9.
Change-Id: Ie9be1eb3071abcc2df7723ceb10d76fe458c4808