build-tools to 31.0.0; SDK cmd-line tools to 5.0. (#174)

* Update build-tools to 31.0.0. Add API 31 emulator to workflow. Update SDK command-line tools to 5.0

* Update test fixture dependencies.

* Test API 31 with macos-11.

* Add android:exported to AndroidManifest.

* Format

test-fixture/app/src/main/AndroidManifest.xml

@@ -9,7 +9,9 @@
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/AppTheme">
-        <activity android:name="com.example.testapp.MainActivity">
+        <activity
+            android:name="com.example.testapp.MainActivity"
+            android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
 

test-fixture/app/src/main/res/layout/activity_main.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
+<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
@@ -10,9 +10,6 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="Hello World!"
-        app:layout_constraintBottom_toBottomOf="parent"
-        app:layout_constraintLeft_toLeftOf="parent"
-        app:layout_constraintRight_toRightOf="parent"
-        app:layout_constraintTop_toTopOf="parent" />
+        android:layout_gravity="center" />
 
-</androidx.constraintlayout.widget.ConstraintLayout>
+</FrameLayout>