Enable fs-verity to all APKs on install

Previously, we only enable fs-verity to an APK if it comes with a
trusted signature (.fsv_sig). With this change, we'll enable fs-verity
in integrity-only mode if there's no signature.

The biggest benefit is O(1) measurement of the APK content, and can be
useful to some use cases.

Note that integrity-only does not imply security, since without a
signature, an attacker can also enable fs-verity on arbitrary files.

Bug: 249158715
Test: CtsAppSecurityHostTestCases:android.appsecurity.cts.ApkVerityInstallTest
Change-Id: I119e5189603af888dfa1ece2bee9e7635120854b