Introduce IMM#getCurrentInputMethodInfo(userId)

introduce an API for apps to query currently selected aka default input
method for current or a given user.
Existing API getEnabledInputMethodList() builds list for allenabled
IMEs, which is inefficient and redundant for several cases.

Bug: 255799034
Test: atest CtsInputMethodServiceHostTestCases:MultiUserTest
Change-Id: I60a0f67bf7d261d3a4a733adcb8a022ceac6e1db