Support encryption hook

Problem Statement

If events contain sensitive data, and the data is stored in the device, the device is sandboxed. Still, if you want everything to be encrypted, you can encrypt events before flushing them to the disk and decrypting them before sending them to the network.

Solution Brainstorm

https://posthog.com/docs/libraries/android#all-configuration-options

// Hook for encrypt and decrypt events
// Devices are sandbox already
// Defaults to no encryption
encryption = object : PostHogEncryption { ... }

Start with the linked Android configuration documentation, then inspect the SDK's event persistence and network-send entry points; no Flutter files or tests are named in the issue. Define an optional encryption/decryption hook for events, preserving unencrypted behavior by default, and verify that events are encrypted before disk flushing and decrypted before network sending.