apex mixed builds: use the .capex when required.

The .capex is the compressed apex. Depending on various conditions, the
.capex should be installed, and thus the
apex.go#ProcessBazelQueryResponse should set the .capex as the main apex
output file.

Fixes: 264873511
Test: new unit test with install rules
Test: m com.android.adbd --bazel-mode-staging gives:

$ ls -lah
out/target/product/generic_arm64/system/apex/com.android.adbd.capex
-rw-r----- 1 jingwen primarygroup 3.6M Jan 10 14:44 out/target/product/generic_arm64/system/apex/com.android.adbd.capex

Change-Id: Ic3143cc6999f111428c6adb76760be8ddc4b6344