I think you already know that Nexus 7 (2012) Android 5.0 factory image is out. Anyone can download it from here and isntall it.
If you have successfully flashed it to your Nexus and if you are not seeing "Sinhala" characters properly you can enable it from the following method.
If you haven't already root the new ROM
Goto CF-Auto-Root beta download site
Download correct boot image. You will find a zip file there.
Turn off your device. Goto boot loader by holding volume down + power
Connect the USB cable.
Unzip it. Run
For Linux : root-linux.sh
For Mac : root-mac.sh
For Windows root-windows.bat
Your device will restart several times and install SuerpSU (Root utility) after final successful boot.
Now your device is rooted.
Download Sinhala fonts from Google noto fonts
You'll get a zip file which has two .ttf Sinhala fonts.
(NotoSansSInhala-Regular.ttf and NotoSansSInhala-Bold.ttf)
In order to add Sinhala font files you'll need a file browser which has root access capability.
I used ES file explorer. Find it on Google Play
In ES file explorer you have to open left drawer and enable Root Explorer feature to access system files.
Copy two .ttf files which you downloaded earlier and copy them to the device
Now reboot the devices and you'll see proper Sinhala character rendering.