Android: select a pbuffer config for fallback surfaces

A window config need not support pbuffers. Creating the
fallback with that config can fail with EGL_BAD_MATCH even
when a compatible pbuffer config is available.

Reuse the existing EGL context helper to select a pbuffer
config from the actual context format. Preserve fallback
caching and diagnostics and let makeCurrent report binding
failures.

An isolated production-method regression fails with the old
window-only config and passes with this change. Android 17
passes fallback-to-window rendering and the complete native
surface suite with both containers. Incompatible physical
driver coverage remains unavailable.

Pick-to: 6.12
Change-Id: I80c8dbeeef07b012d88428bed6dc5ca0b49775cb
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>