Add environment variable BUILD_FROM_SOURCE_STUB

On top of the soong build flag --build-from-source-stub, the user can
configure build to generate stubs from source Java files by setting the
environment variable BUILD_FROM_SOURCE_STUB=true.

Test: m nothing
Bug: 310279899
Change-Id: I0c5247558455f09b8fc208d988aab3b60bf0ff16
diff --git a/android/config.go b/android/config.go
index bf0b9de..572e006 100644
--- a/android/config.go
+++ b/android/config.go
@@ -2079,7 +2079,7 @@
 	// TODO: b/302320354 - Remove the coverage build specific logic once the
 	// robust solution for handling native properties in from-text stub build
 	// is implemented.
-	return !c.buildFromSourceStub && !c.JavaCoverageEnabled()
+	return !c.buildFromSourceStub && !c.JavaCoverageEnabled() && !c.IsEnvTrue("BUILD_FROM_SOURCE_STUB")
 }
 
 func (c *config) SetBuildFromTextStub(b bool) {