Allow injecting layers by manifest metadata

Bug: 138204026

Test: Add <meta-data android:name="com.android.graphics.injectLayers.enable" android:value="true" />
to the <application> xml in the manifest of the RELEASE build of a GL/Vk app. Add layer(s) using the method
described here: https://developer.android.com/ndk/guides/graphics/validation-layer

Change-Id: If4809c139f4a667ddf125358274a6d1d030a57d8