Sign apks with apksig v2/v3

(Surprisingly we didn't have a ticket for this yet?)

Android R/11/30 (to be released very soon now) will stop allowing to install apps signed only with v1 signatures when they have targetSdk 30. I just encountered this in an emulator:

adb: failed to install repo/app.olauncher_11.apk: Failure [INSTALL_PARSE_FAILED_NO_CERTIFICATES: Scanning Failed.: No signature found in package of version 2 or newer for package app.olauncher]

!736 (merged) implements signing with apksigner (needs refreshing but generally works). This would easily allow switching to producing v2/v3 signatures but it breaks our reproducible builds processes until we implement a solution for that.

I'd suggest as an interim workaround to enable signing apks with targetSDK >=30 with apksigner ASAP and build the reproducible builds afterwards. When that's done we can switch all new signatures to v2+.