Vending: Fix repeated EMUI install dialog and implement splitDeferred

[grenadin]

Fixes #3541

Problem

On Huawei EMUI devices (Android 12+), two related issues in vending-app
cause InstallDistActivity (Huawei's install confirmation dialog) to appear
repeatedly during app installation via microG Companion installer proxy.

Issue 1 — Missing USER_ACTION_NOT_REQUIRED flag

installPackagesInternal() in Install.kt creates PackageInstaller.SessionParams
without setting USER_ACTION_NOT_REQUIRED. On EMUI, the system intercepts every
session commit and routes it through InstallDistActivity, showing a mandatory
confirmation dialog — including for background DFM installs.

Issue 2 — splitDeferred() was a no-op stub

SplitInstallService.splitDeferred() only logged a warning and ACKed immediately
without performing any actual installation. Apps such as Facebook request background
pre-installation of DFM splits (e.g. pytorch, papaya, executorch) via
splitDeferred(). Because nothing was installed, the app fell back to startInstall()
for the same modules — triggering additional PackageInstaller sessions and additional
InstallDistActivity dialogs per module.

Context

SplitInstallService and splitDeferred() were originally introduced in #2500
(Implement SplitInstallService). At that time splitDeferred() was left as a stub —
it only logged "not implemented" and never called back, which is the root cause of
Issue 2 above. This PR implements it by routing through the same splitInstallFlow()
already used by startInstall(), rather than introducing a new code path.

Fix

Install.kt — Add USER_ACTION_NOT_REQUIRED to SessionParams for API 31+:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
    params.setRequireUserAction(SessionParams.USER_ACTION_NOT_REQUIRED)
}

SplitInstallService.kt — Route splitDeferred() through splitInstallFlow()
(same as startInstall()), then ACK with onDeferredInstall() after completion
instead of being a no-op.

Testing

Tested on Huawei MatePad Mini (EMUI 12, Android 12) with microG Companion
0.3.15.250932-hw built from this branch.

Test steps:

1. Install Aurora Store → select microG as installer method
2. Install Facebook (com.facebook.katana) and TikTok (com.tiktok)
3. Open each app and allow DFM modules to load

Before fix (logcat evidence in #3541):

• InstallDistActivity appeared as topActivity before every session commit
• splitDeferred() called for com.facebook.katana, but is not implemented (repeated per module)
• App fell back to startInstall() for same DFM modules → additional dialogs

After fix:

• InstallDistActivity no longer appears as topActivity ✅
• splitDeferred not implemented no longer logged ✅
• Facebook and TikTok DFM modules install silently in background ✅

Notes

• USER_ACTION_NOT_REQUIRED requires API 31 (Android 12+), guarded by
  Build.VERSION.SDK_INT >= Build.VERSION_CODES.S
• splitDeferred() ACKs with onDeferredInstall(Bundle()) regardless of
  install result so the calling app is never left waiting
• This fix matches the behavior already implemented in Aurora Store's
  SessionInstaller.kt for its own session installer path
• Related: PR #3074 (App Installer Proxy)
• Builds on the SplitInstallService skeleton introduced in Implement SplitInstallService #2500

Full logcat after fix (Facebook + TikTok):
facebook_log_after_fix_redacted.txt