Clean up setInputChannelLocked

-. Rename setInputChannelLocked and add a static
   method for checking same InputChannel.

Suppose we need to handle the flush pending events
when the input channel is modified based on CL[1],
CL[2] and CL[3].

This is a mechanical refactor and it should no
user visible change.

[1]: Ibe26311edd0060cdcae80194f1753482e635786f
[2]: I7e0b6a3c43cbe72f8762991f5d36560feebd214b
[3]: If8317941dd1c6d5db77f1239b9a9f45d49997df9

Bug: 236920321
Test: atest CtsInputMethodTestCases
Change-Id: I4f0d2404705a8f93de345571136690dd408d1132