[3.14] gh-146450: Ensure Android gradle build uses custom cross-build dir (GH-148319) (#148470)

Ensures that the testbed's Gradle configuration uses the cross-build environment
variable, and that variable is passed to Gradle by the cross-build script.
(cherry picked from commit b29afe62f7236f7161c2670dccc24368217a7fb1)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>
Co-authored-by: Malcolm Smith <smith@chaquo.com>