Fix the detection for compressed APKs.

We pack _all_ the APK certs info into META/apkcerts.txt at build time,
including the ones that are not installed for the target. When
detecting for compressed APKs, we should only count the ones that are
actually installed.

Bug: 65498015
Test: `check_target_files_signatures.py bullhead-target_files.zip`
Change-Id: I2f32d4667be0efbfd5f9365dde819f009572533d