[Amazon TV] change manifest to allow installation to 'external storage'

At the moment, RetroArch installs only to internal memory on Amazon Fire devices, and does not allow the user to move it to attached external storage. This is an issue because the built-in storage is pretty limited.

From my very cursory reading, this should be as simple as a change in the manifest file that is being provided to Amazon. Here is a related section from the Fire tablet developer docs, which I would assume apply to Fire devices across the board : https://developer.amazon.com/docs/fire-tablets/ft-specify-installation-location.html

The settings in your Android Manifest file determine where your app is installed on Fire devices. Two storage locations are possible:

   * External storage (such as an SD card)
   * Internal storage (using the device's memory)

Although older Fire tablets don't have external storage, the newer Fire tablets provide external storage options through memory card slots.

Generally, your app should specify external storage as the default install location.

The docs go on to provide sample code for the manifest. "In the AndroidManifest.xml of your app, inside the manifest tag, add the installLocation attribute and set its value to preferExternal. That tag indicates to install your app on external storage when available. If external storage is full, install the app to internal storage (if available). The user can manually move the app later by selecting it from Settings > Apps & Games > Manage All Applications."

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    android:installLocation="preferExternal" ... >
    ...
</manifest>

Start with the AndroidManifest.xml used for the Amazon Fire build and review Amazon's installation-location documentation linked in the issue. Confirm the manifest supports preferred external installation, then verify on a Fire device that the app can install to or be moved to available external storage.