Add run-android-variant-matrix.sh so smoke covers F-Droid stable APK,
a self-build of the same release tag (wallet-1.6.1), and origin/master.
Build helper accepts tags; avoids relying on a single developer tree.
Default EMULATOR_HEADLESS=1 (-no-window, QT offscreen) while guest GLES
still runs via SwiftShader. Auto-start AVD when no adb device; WINDOWED=1
for a host window on laptops.
