Make MULTI_CLIEN_IME_ENABLED non-testable.

This mechanically reverts making MULTI_CLIEN_IME_ENABLED
testable originally introduced in [1].
CTS tests can check for FEATURE_INPUT_METHODS instead.
FEATURE_INPUT_METHODS is false for Android auto and true for
devices that support installable IMEs.

[1] Id7b4dceb2dbfaf3d7ed1084957dd14b04cad0cdf

Bug: 137203665
Test: Manually built and verified that it builds.
Change-Id: I6e430458dd05dec363f67b62416d424c395f2d5a