VEX-6030: Reduce buffer size based on heap (#13)

This PR changes the behavior on old devices that have poor memory management.

Jira: VEX-6030
https://jira.tenkasu.net/browse/VEX-6030

The solution implied customizing the method shouldContinueLoading from RNVLoadControl to use only the available heap, performing tests on an old Nexus 5 it was determined the ideal bytes allocation to half the reported heap, that provided some buffering during ads but smooth playback during the video with no crashes (23:39 of 23:39 at the moment of writing this, video kept playing as expected after 3 ad breaks)

The fix is only targeting Marshmallow as the reduction of buffer is substantial and other versions that work properly should not get affected.

Depending on the test result of VEX-5758, this same fix can be applied to Nougat

Reviews
Major reviewer (domain expert): @armadilio3
Minor reviewer: @nickfujita

Video.js

@@ -468,6 +468,7 @@ Video.propTypes = {
     maxBufferMs: PropTypes.number,
     bufferForPlaybackMs: PropTypes.number,
     bufferForPlaybackAfterRebufferMs: PropTypes.number,
+    maxHeapAllocationPercent: PropTypes.number,
   }),
   stereoPan: PropTypes.number,
   rate: PropTypes.number,