Bump minimum minSdkVersion to 19.

We need to land an update to the NDK prebuilts and the NDK no longer
supports APIs 16-18.

Bug: http://b/222341313
Test: treehugger
Change-Id: I996b0eb65a7d1ae4cc0687b7ed8f533fbbba295d
diff --git a/android/config.go b/android/config.go
index cb2fc61..9f1fd6a 100644
--- a/android/config.go
+++ b/android/config.go
@@ -778,7 +778,7 @@
 }
 
 func (c *config) MinSupportedSdkVersion() ApiLevel {
-	return uncheckedFinalApiLevel(16)
+	return uncheckedFinalApiLevel(19)
 }
 
 func (c *config) FinalApiLevels() []ApiLevel {