Remove getSourceConsumer special case for IME

Previously there was a cyclic dependency in getSourceConsumer, as it
reads the value of mImeSourceConsumer, but it is also used to initialize
that field.

This removes the special case for IME from getSourceConsumer, as we
always add the IME to the mSourceConsumers array, and we never remove
it. If the type of the consumer is ime, the id must be ID_IME, so this
will always retrieve the same already-created instance.

This also allows making the ID field in InsetsSourceConsumer final.

Flag: EXEMPT bugfix
Test: n/a
Bug: 281029564
Change-Id: Ib197e50bcb1059d3ae9d9ee0d47a2fab1b1a57d0