Added caching to the gradle properties

Also added android:exported tags so that android:processDebugMainManifest doesn't fail the gradle check.
I think this is only failing locally because I bumped the compileSdkVersion to 31, but that means it'll need to go in soon anyway.

android/AndroidManifest.xml

@@ -21,6 +21,7 @@
         <activity
             android:name="com.unciv.app.AndroidLauncher"
             android:launchMode="singleTask"
+            android:exported="true"
             android:label="@string/app_name"
             android:configChanges="keyboard|keyboardHidden|orientation|screenSize"
             tools:ignore="LockedOrientationActivity">
@@ -34,6 +35,7 @@
         <activity
             android:name="com.unciv.app.AndroidTvLauncher"
             android:label="@string/app_name"
+            android:exported="true"
             android:theme="@style/GdxTheme">
 
             <intent-filter>

gradle.properties

@@ -1,3 +1,4 @@
 android.useAndroidX=true
 android.enableJetifier=true
-org.gradle.parallel=true
+org.gradle.parallel=true
+org.gradle.caching=true