fix: improve Android Base64 and Ed25519 key export compatibility - #285

This fixes two compatibility issues found while migrating ConnectBot to cbssh.

• Fall back to java.util.Base64 only when invoking android.util.Base64 through reflection throws InvocationTargetException. Android Base64 remains the preferred path; this supports JVM/Robolectric environments where its implementation is present but unusable through reflection.
• Export PKCS#8-encoded Ed25519 private keys without relying on a provider-specific class name. This enables OpenSSH export for Conscrypt keys while invalid or unsupported encodings continue to fail with SshException.

No public API changes.

Validation:

• ConnectBot OSS and Google debug Kotlin compilation
• ConnectBot unit-test suite