Enable gradle dependency verification

[emanuelb]

This is done by having ./gradle/verification-metadata.xml file, more information:
https://docs.gradle.org/current/userguide/dependency_verification.html

[Tougee]

I haven't seen any Android projects that use this specification, so won't integrate it now.

[emanuelb]

I haven't seen any Android projects that use this specification, so won't integrate it now.

Android Bitcoin Wallets that implement this:

1. Green wallet by Blockstream:
   https://github.com/Blockstream/green_android/blob/master/gradle/verification-metadata.xml
   See also related script they developed:
   https://github.com/Blockstream/green_android/blob/master/update_dependency_verification.sh

2. SBW (Simple Bitcoin Wallet)
   https://github.com/btcontract/wallet/blob/master/gradle/verification-metadata.xml