Custom soft keyboard for sinhala language

I'm developing a custom keyboard for android to use Sinhala language. since Android 5.0 onwards it has native support for Sinhala language.

I'm using Unicode chart form here. as you can see Sinhala language use vowel signs like "ා", "ැ" and "ු". and letters combined with those signs make different pronunciations for same letter like "ත+ා = තා", "ත+ැ = තැ" and "ත+ු = තු" similar in English "Tha", "Thae" and "Thu".

My problem here is most of the letters I input from keyboard rendered in a textbox like it intended to but when I use vowel signs they just replace all the text in textbox from that sign and does not make that combined letter I expect.

I'm binding keys to keyboard layer in following way.

<Row>
    <Key android:codes="3540" android:keyLabel="ු" android:keyEdgeFlags="left"/>
    <Key android:codes="3461" android:keyLabel="අ"/>
    <Key android:codes="3536" android:keyLabel="ැ"/>
    <Key android:codes="3515" android:keyLabel="ර"/>
    <Key android:codes="3474" android:keyLabel="ඒ"/>
    <Key android:codes="3524" android:keyLabel="හ"/>
    <Key android:codes="3512" android:keyLabel="ම"/>
    <Key android:codes="3523" android:keyLabel="ස"/>
    <Key android:codes="3503" android:keyLabel="ද"/>
    <Key android:codes="3488" android:keyLabel="ච"/>
    <Key android:codes="3492" android:keyLabel="ඤ" android:keyEdgeFlags="right"/>
</Row>

So I want to know how can I insert the those combined letters as I intended to.