Make robolectric runtimes a host module

I931f9145f1ced21a9f86d647cfbb6d2b645b4f18 made robolectric_android-all-stub
empty, breaking running robolectric tests packaged by Soong.
To enable using robolectric-host-android_all instead, make the
runtimes module a host module.

Also remove the empty robolectric_android-all-stub dependency.

Test: m checkbuild
Change-Id: Idc158d84fd8b6d98d824a6bdc8be297ac43ec2f2