Don't dexpreopt when compile_dex but not installable

Modules shouldn't be dexpreopted and possibly stripped when
compile_dex is set but installable is not set.  This matches the
previous behavior when the dexpreopt rules were in Make.

Bug: 121317615
Test: dexpreopt_test.go
Change-Id: I6f80b7b37a990c475b91aa5d98a19a3baa85eb1b