camera: Rewrote Android support.

This does something a little weird, in that it doesn't care what
`__ANDROID_API__` is set to, but will attempt to dlopen the system
libraries, like we do for many other platform-specific pieces of SDL.

This allows us to a) not bump the minimum required Android version, which is
extremely ancient but otherwise still working, doing the right thing on old
and new hardware in the field, and b) not require the app to link against
more libraries than it previously did before the feature was available.

The downside is that it's a little messy, but it's okay for now, I think.

android-project/app/src/main/AndroidManifest.xml

@@ -37,6 +37,13 @@
         android:name="android.hardware.microphone"
         android:required="false" /> -->
 
+    <!-- Camera support -->
+    <!-- if you want to record video, uncomment this. -->
+    <!--
+    <uses-permission android:name="android.permission.CAMERA" />
+    <uses-feature android:name="android.hardware.camera" />
+    -->
+
     <!-- Allow downloading to the external storage on Android 5.1 and older -->
     <!-- <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="22" /> -->