Drop support for KitKat.

The NDK is dropping support for this.

Bug: https://github.com/android/ndk/issues/1751
Test: treehugger
Change-Id: I6c03dfc364e7612ce4f45fb58746726983cf5b74
diff --git a/android/config.go b/android/config.go
index d3f8ab4..15707a5 100644
--- a/android/config.go
+++ b/android/config.go
@@ -689,7 +689,7 @@
 }
 
 func (c *config) MinSupportedSdkVersion() ApiLevel {
-	return uncheckedFinalApiLevel(19)
+	return uncheckedFinalApiLevel(21)
 }
 
 func (c *config) FinalApiLevels() []ApiLevel {