Minimum requirement of all variants of GeckoView changes to Android 5 (API 21)

[Makoto Kato]

Hello,

GeckoView has two variants that are GeckoView and GeckoView Lite. The
minimum requirement of GeckoView Lite that had no Glean dependencies
was Android 4.1 (API 16) on 32-bit (ARM/x86).

I have landed bug 1820295 (https://bugzil.la/1820295), so the minimum
requirement of all variants of GeckoView changes to Android 5 (API 21)
from 119.

-- Makoto