Don't panic when DevicePrimaryArchType is called in a host only build

Check the length of c.Targets[Android] before getting the first
element, and return Common if the length is set.  Fixes a panic
when a tradefed_binary is defined in a host-only build.

Test: aosp-build-tools build
Change-Id: I89f1dd13aa1aff5eb7d39e0b66eeedcb7d930fa0