Make MultiClientInputMethod optional build target

Otherwise SDK build will fail because this target puts artifacts to
privileged app dir, which is not expected for samples in the SDK
build.

You can still build MultiClientInputMethod on command line by
explicitly specifying this in the build target.

Bug: 115784148
Test: locally verified as follows:
  lunch sdk_phone_armv7-userdebug
  make -j PRODUCT-sdk_phone_armv7-sdk
Change-Id: I09282fa39d13f8078881a3b215cad7d3c0e81486