Make a class initialization deadlock more impossible

The following field create a class initialization cycle, since `Base64Encoding` extends `BaseEncoding`:

```
  private static final BaseEncoding BASE64_URL = new Base64Encoding(...);
```

In practice this example is probably harmless, because `Base64Encoding` is already package-private and there are no references outside this file. Making it explicitly private makes it more obviously correct (to Error Prone, and hopefully also to human readers), since it ensures the only way to get an instance of `Base64Encoding` is to first initialize `BaseEncoding`.

See also https://errorprone.info/bugpattern/ClassInitializationDeadlock

RELNOTES=N/A
PiperOrigin-RevId: 616154233

android/guava/src/com/google/common/io/BaseEncoding.java

@@ -611,7 +611,7 @@ public int hashCode() {
     }
   }
 
-  static class StandardBaseEncoding extends BaseEncoding {
+  private static class StandardBaseEncoding extends BaseEncoding {
     final Alphabet alphabet;
 
     @CheckForNull final Character paddingChar;
@@ -948,7 +948,7 @@ public int hashCode() {
     }
   }
 
-  static final class Base16Encoding extends StandardBaseEncoding {
+  private static final class Base16Encoding extends StandardBaseEncoding {
     final char[] encoding = new char[512];
 
     Base16Encoding(String name, String alphabetChars) {
@@ -995,7 +995,7 @@ BaseEncoding newInstance(Alphabet alphabet, @CheckForNull Character paddingChar)
     }
   }
 
-  static final class Base64Encoding extends StandardBaseEncoding {
+  private static final class Base64Encoding extends StandardBaseEncoding {
     Base64Encoding(String name, String alphabetChars, @CheckForNull Character paddingChar) {
       this(new Alphabet(name, alphabetChars.toCharArray()), paddingChar);
     }

guava/src/com/google/common/io/BaseEncoding.java

@@ -611,7 +611,7 @@ public int hashCode() {
     }
   }
 
-  static class StandardBaseEncoding extends BaseEncoding {
+  private static class StandardBaseEncoding extends BaseEncoding {
     final Alphabet alphabet;
 
     @CheckForNull final Character paddingChar;
@@ -948,7 +948,7 @@ public int hashCode() {
     }
   }
 
-  static final class Base16Encoding extends StandardBaseEncoding {
+  private static final class Base16Encoding extends StandardBaseEncoding {
     final char[] encoding = new char[512];
 
     Base16Encoding(String name, String alphabetChars) {
@@ -995,7 +995,7 @@ BaseEncoding newInstance(Alphabet alphabet, @CheckForNull Character paddingChar)
     }
   }
 
-  static final class Base64Encoding extends StandardBaseEncoding {
+  private static final class Base64Encoding extends StandardBaseEncoding {
     Base64Encoding(String name, String alphabetChars, @CheckForNull Character paddingChar) {
       this(new Alphabet(name, alphabetChars.toCharArray()), paddingChar);
     }