Evaluate utility of v4 signatures for Accrescent

[lberrymage]

Accrescent is currently completely oblivious to APK signature scheme v4. However, it may be useful for us to utilize v4 signatures for stronger signature verification features.

Browsing AOSP source code seems to suggest that adding v4 signature files to normal installation sessions with names in the form ${apkName}.idsig enables fs-verity for those APKs (at least as of the latest version of Android 16 published to AOSP as of the time of writing). The GrapheneOS App Store applies them to the install session this way, and this code claims that v4 signatures are used by the OS to enable fs-verity for APKs on SDK 35+. The GrapheneOS App Store supports non-GrapheneOS operating systems where it isn't integrated as a privileged installer, so there's an implication that fs-verity may work for any AOSP-based OS.

Platform behavior should be thoroughly investigated to determine whether Accrescent gains any benefit (e.g., continuous verification for apps installed through Accrescent by opting them in to fs-verity participation) by passing v4 signatures to its install sessions on Android proper as well as other AOSP-based platforms.

If utilizing v4 signatures is possible and beneficial to Accrescent, implementing support depends on #53 since we don't currently accept v4 signature uploads through the Accrescent developer console and #53 would make it convenient for developers to submit them.

[lberrymage]

We have confirmation that adding v4 signature files to installation sessions does enable fs-verity for the APKs. However, it isn't really enforced in AOSP or Android. We may as well add support for v4 signatures though because 1) doing so may be useful in future releases of Android and 2) there may be some use we can get out of them today in GrapheneOS because of some security enhancements they make to this part of the OS.