[Android] The device heats up quite much while using the "Notes" application for a period of time

[Softvision-MariusComan]

[Affected Platforms]:

• Android 8.0.0 (Pixel One)
• Android 8.0.0 (Samsung S8)
• Android 6.0 (Huawei P8 Lite)

[Prerequisites]:

• Have the latest version of the "Firefox Notes" application (notes-1_0-qa-v2249 custom built on 22.05.2018) installed on the device (Google Pixel for clear results).

[Steps to reproduce]:

1. Open the Notes application and log in if needed.
2. Use the "Notes" app for ~10 minutes and observe the CPU temperature.
3. Close the app and let the device to cool off.
4. Use the "Keep" note application for ~10 minutes and observe the CPU temperature.
5. Close the app and let the device to cool off.
6. Play a video on "Youtube" for ~10 minutes and observe the CPU temperature

[Expected result]:

• The device heats up in the normal range while using the "Notes" application.

[Actual result]:

• Step 2: The CPU's temp goes from 30°C to 47°C.
• Step 4: The CPU's temp goes from 30°C to 37°C.
• Step 6: The CPU's temp goes from 30°C to 33°C.

[Notes]:

• The Google Pixel device is the most affected by this issue, but it's also noticeable on the Samsung S8 device (41°C) and on the Huawei P8 LITE device(38°C) after 10 minutes of use.
• The Notes application does not heat's up the device while the device is locked.
• Attached a screenshot of the issue:

[vladikoff]

This is probably due to encryption / decryption, there might be some optimizations we can do here...

[vladikoff]

@Softvision-MariusComan , I made some changes, so it could be a bit cooler, but still running hot when encrypting / decrypting...