use apksigner defaults to sign index-v2.jar

The following discussions from !1092 (merged) should be addressed:

@eighthave started a discussion: (+15 comments)

The new signing method should use the default Java settings, since Java and Android both maintain those. That helps avoid a repeat of being stuck on an old signing algorthim. That means specifically that this call to jarsigner does not specify any of the algorithms, e.g. jarsigner -keystore config['keystore'] -storepass:env FDROID_KEY_STORE_PASS index-v2.jar alias
@eighthave started a discussion: (+3 comments)

How about we make this looking towards the future by default, with a flag to enable the old sig?

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