As discussed several times, Android builds take too much time, because any change in GeckoView (or in anything that causes it to be recompiled) results in 4 builds of GV, one for each architecture.
Mozilla supports only multi-arch GV now, and we don't want to depart too much from them, but we'd like also to have our process more streamlined.
Therefore, as a tradeoff, this patch makes building single-arch TBA possible, but it's enabled by default only on testbuilds (see comments on #40116 (closed)).
Supersedes !90 (closed).