Enable background users to call IMM#getCurrentInputMethodSubtype()

This is a follow up CL to my previous CLs [1][2], which let query APIs
defined in InputMethodManager support background users.

With this CL,

  InputMethodManager#getCurrentInputMethodSubtype()

is also fully supported under multi-user / multi-profile environment.

 [1]: I192a0f5a1375170d17a4c08af94f23966dbaea8b
      7f8ee4b9
 [2]: I48f57dc7184e85bdb422fd9d1d56e60381654125
      e9609865

Bug: 34886274
Bug: 122164939
Bug: 237316307
Test: atest CtsInputMethodTestCases:InputMethodSubtypeTest
Change-Id: Icb09f9cb1a4147884faa9952b2e03ec4afa9f0b1