Auto merge of #12261 - szeged:gattcharacteristicfunctions, r=jdm

Missing steps of Characteristic's readValue, writeValue functions

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Add a check for the read property of the characteristic as described in https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothremotegattcharacteristic-readvalue (Step 4.1)
Add two missing steps to characteristic's WriteValue function. https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothremotegattcharacteristic-writevalue (Step 4 and 5)

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because, there are no Web Bluetooth test API implementation yet.

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