Add noOverrideExternalGlobalCflags support to Soong

noOverrideExternalGlobalCflags is a new set of flags intended to be
added at the end of the command line for 3rd party projects like
external/, hardware/, vendor/, and more. Our previous flags for external
projects occur way too early on the command line, leading to issues with
use for suppressing diagnostics. Note that support for this variable in
build/make is currently unimplemented, as there are no projects that
would depend on it. It could be added in the future, if it turns out to
be useful.

This change initially applies `-Wno-unused-but-set-variable` and
`-Wno-unused-but-set-parameter` to external projects because these
upstream projects aren't easily fixed, and the risk of actual bugs is
low.

Bug: http://b/197240255
Test: Build and check warnings
Change-Id: I26f56c5c52725dddb70dd8130ad61270eac7a9aa