[BOUNTY #2851] Remote DroidGuard server guide + multi-step session support

[Karan02204]

Problem
Remote DroidGuard previously only supported single-step flows. Play Integrity (and related flows) use a multi-step DroidGuard process involving init/initWithRequest, multiple snapshot calls, and session state.

As a result, setting DroidGuard to Network (remote) mode did not work for Play Integrity. Users were forced to run root + bypass modules (TrickyStore, PlayIntegrityFix, etc.) on their daily driver.

This was the core technical blocker described in #2851.

Solution
This PR fully enables remote DroidGuard for Play Integrity by improving the remote client implementation.

Key Changes

• Reworked RemoteHandleImpl.kt:

• Full IDroidGuardHandle lifecycle: init, initWithRequest, snapshot, close

• Session ID (sid) management for multi-step flows

• Proper forwarding of DroidGuardResultsRequest.bundle (supports String, ByteArray, lists, etc.)

• Added protocol support for action=init and action=close

• Robust payload encoding + response parsing (including sid|result format)

• Graceful fallback to stateless mode

Play Integrity flows now work over remote:

• pia_attest_e1
• pia_express
• Other multi-step flows
• All high-level callers (DroidGuard.getClient().getResults(...), IntegrityService, ExpressIntegrityService, PoTokenHelper, etc.) automatically benefit when remote mode is enabled.

Documentation
Added comprehensive documentation:

docs/remote-droidguard.md — complete guide covering:

• Client setup (microG Settings)
• Full remote protocol specification
• Reference server implementation examples
• How to run on stock phones vs custom-ROM server devices
• Testing, security, and limitations

docs/remote-droidguard-server-example.kt — ready-to-adapt server code
Impact

• Enables offloading Play Integrity attestation to another device (stock phone, home server, or commercial service).
• Client devices no longer require root or constant bypass maintenance.
• Opens the path for commercial "integrity as a service" offerings.
• Fixes related issues (Firebase App Check, Dott, banking apps, etc.) on microG/custom ROM devices when using a passing remote backend.

Related

• Addresses the main technical requirement in [BOUNTY] Support Play Integrity over remote DroidGuard + Server/Guide [$35] #2851
• Builds on previous remote DroidGuard infrastructure

Notes

• This PR delivers the client-side fix (the difficult part).
• A full production remote DroidGuard server app is still recommended as follow-up work (protocol + examples are included).
• The legacy guardWithRequest direct callback path still contains a TODO (not used by current Play Integrity code paths).