Emoji are invisible on some devices when using LAYER_TYPE_NONE

This issue can be reproduced on master with the Samsung Galaxy Ace 2 (Android 4.1.2) by creating a large EmojiTextView (for example a long forum post), so that rendering switches from LAYER_TYPE_SOFTWARE to LAYER_TYPE_NONE. In the screenshot below, the post at the top of the screen is below the threshold and uses LAYER_TYPE_SOFTWARE, while the invisible post filling the rest of the screen is above the threshold and uses LAYER_TYPE_NONE.

The issue can be reproduced with EmojiTextViews of any size by changing the code to always use LAYER_TYPE_NONE. This makes it easier to see that emoji are sometimes drawn and sometimes not. For example, the screenshot below shows a mixture of posts, all of which use LAYER_TYPE_NONE and contain strings of emoji.

The issue also affects EmojiEditText if the rendering type is changed to LAYER_TYPE_NONE. In the screenshot below, one emoji has been typed but it's invisible.

Selecting invisible emoji makes them visible.

The issue also occurs on master with the Huawei Ascend Y330 (Android 4.2.2).

If an EmojiTextView contains a mixture of emoji and text, the text is rendered normally. In the following screenshot, the post starts with one emoji, which is invisible.