Add minSdkVersion=21 to ABI split manifests

Appears to be required for split-select to work

BUG=484797

Review URL: https://codereview.chromium.org/1142893005

Cr-Original-Commit-Position: refs/heads/master@{#330615}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 15390e42957d99e29177576d91d77cb3053a52aa

android/gyp/generate_split_manifest.py

@@ -19,6 +19,7 @@ MANIFEST_TEMPLATE = """<?xml version="1.0" encoding="utf-8"?>
     xmlns:android="http://schemas.android.com/apk/res/android"
     package="%(package)s"
     split="%(split)s">
+  <uses-sdk android:minSdkVersion="21" />
   <application android:hasCode="%(has_code)s">
   </application>
 </manifest>