Prepare IME ConcurrentMultiUserTest to move to cts

This changelist prepares the Concurrent Multi-User IME tests to be moved
from fw/base to cts/tests/inputmethod. It renames the test activity and
resources to avoid conflicts with the already existing ones in IME CTS.

These tests have been consistently passing for some time and are now
mature enough for inclusion in CTS. In CTS, they will execute
exclusively on devices supporting concurrent multi-user IME.

Bug: 401547377
Test: atest ConcurrentMultiUserTest
Flag: TEST_ONLY
Change-Id: Ia90fd88a5cd06fe5f1e8570a44e4936e062829eb