Replace remaining usages of Guava's `BaseEncoding`

As of today we are using:

* `BaseEncoding.base16()` -- can be replaced with our existing `ByteUtils` methods: `formatHex` and `parseHex` -- see PR #4011
* `BaseEncoding.base32()` -- can be replaced with Bouncy Castle [Base32](https://downloads.bouncycastle.org/java/docs/bcprov-jdk15to18-javadoc/org/bouncycastle/util/encoders/Base32.html) -- see PR #4012
* `BaseEncoding.base64()` -- can be replaced with ~~Bouncy Castle [Base64](https://downloads.bouncycastle.org/java/docs/bcprov-jdk15to18-javadoc/org/bouncycastle/util/encoders/Base64.html)~~ update: we should use [java.util.Base64](https://docs.oracle.com/javase/8/docs/api/java/util/Base64.html) -- see PR #4017

This helps implement Issue #2105 and although we want to also migrate away from Bouncy Castle (Issue #3912) migrating away from Guava is a short-term goal so I think it is OK to use Bouncy Castle (for now) to achieve it.

**Update:** for the Base32 and Base64 implementations, create methods that are identical (or very similar) to the Guava methods and place them in a class named BaseUtils in `org.bitcoinj.core.internal`.

I would also recommend doing this as a series of 3 PRs: one each for base16, base32, and base64.

**Update:** it looks like we can use `java.util.Base64` as it is available in both Java 8 and Android API 26 (Android 8.0) -- see Android reference documentation: [java.util.Base64](https://developer.android.com/reference/java/util/Base64)

Search the repository for remaining usages of Guava's BaseEncoding, then review PRs #4011, #4012, and #4017 for the intended replacements. Implement the Base32 and Base64 compatibility methods in org.bitcoinj.core.internal.BaseUtils and handle base16 with ByteUtils; done means all remaining BaseEncoding usages are migrated without changing behavior.

Written by the indexing model from the issue text.