Fix multidex artifactId.

In ag/1971845 we accidentally changed artifactId of multidex and
multidex-instrumentation to support-multidex and support-multidex-instrumentation.
This change undoes that change.

Bug: 36122649
Test: ./gradle clean dist now produces correct repo zip
Change-Id: I110446366250315be0d04929b333d47c3fba0d2c