Merge "Add instructions to generate java keystore" into main

This is called by build/make/core/Makefile to incorporate the OTA signing keys

into the recovery image.

converting to java keystore for Android Studio

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Suppose we want to convert shared.pk8 and shared.x509.pem to shared.keystore.

 $ openssl pkcs8 -inform DER -nocrypt                      \

   -in build/make/target/product/security/shared.pk8       \

   -out shared.pem

 $ openssl pkcs12 -export                                  \

   -in build/make/target/product/security/shared.x509.pem  \

   -inkey shared.pem -out shared.p12                       \

   -password pass:android -name AndroidDebugKey

 $ keytool -importkeystore -deststorepass android          \

   -destkeystore shared.keystore -srckeystore shared.p12   \

   -srcstoretype PKCS12 -srcstorepass android

The keystore can be used in build.gradle as follows.

signingConfigs {

    shared {

        storeFile file("shared.keystore")

        storePassword "android"

        keyPassword "android"

        keyAlias "AndroidDebugKey"

    }

}