camera: Make readoutTimestamp visible to HALs

`readoutTimestamp` metadata key was previously only visible to
SDK and the framework, and the camera framework force set the values
based on if the HAL implemented the AIDL or the HIDL interface.

This CL makes readoutTimestamp available to HALs and lets AIDL
HALs choose if they can support sensor readout timestamp or not.

Bug: 309543399
Test: External Camera HAL now sets this value and passes VTS
Change-Id: Ifee2b4020b7630383dc32ef7590f8a9ccdaf49d2