Query right user's enabled IMEs in KeyguardPasswordView

This CL fixes a regression introduced by my previous CL [1], which
enabled InputMethodManager#getEnabledInputMethodList() to return the
result based on the caller's user ID, not based on the current IME
user, even when it gets called from background users.

Since Keyguard always runs as user 0 currently, it is now Keyguard's
responsibility for querying enabled IMEs with an explicit user ID.  To
do so this CL adds a @hide API IMM#getEnabledInputMethodListAsUser()
and lets KeyguardPasswordView use it.

 [1]: I192a0f5a1375170d17a4c08af94f23966dbaea8b
      7f8ee4b9

Bug: 122164939
Fix: 123904896
Test: Manually verified as follows.
  1. Build aosp_taimen-userdebug and flash it.
  2. make -j SoftKeyboard
  3. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  4. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  5. adb shell pm create-user test_user
  6. adb shell am switch-user 10
  7. adb shell locksettings set-password aaaa
  8. adb shell wm dismiss-keyguard
  9. Make sure that the IME switcher icon is not shown at the right
     end of the password field.
Change-Id: I6e7d7353c2b5b1da5d460ae005fb2585f85fb1c4