Enable immersive mode on the legacyXr build

Gates flip from BuildConfig.MODERN_XR to BuildConfig.XR_BUILD (true on both XR
flavors, false elsewhere), and legacyXr gets the same packaging modernXr has:
libxrimmersive.so and the manifest entries for ImmersiveXrActivity and the VR
uses-features. The runtime isMetaQuest check still applies, and the checked-in
libopenxr_loader.so in main jniLibs already ships on legacyXr.

Immersive launch is default-on for legacyXr, matching modernXr; the per-game
checkbox opts out. Note the .so is arm64-only — on an armeabi-v7a install the
loadLibrary failure is caught and the launch degrades to a normal flat start.