MicroBitDisplay: move display refresh timer ownership to MicroBit

[MannuLambrichts]

Summary

This pull request updates how the display refresh timer is instantiated and owned within the Codal display stack. Instead of allocating the timer inline inside the MicroBitDisplay constructor, the timer is now created in MicroBit and passed into MicroBitDisplay by reference, aligning it with how other Codal timers (system, adc, touch) are managed.

Motivation

Previously, MicroBitDisplay allocated the display refresh timer using new within its constructor. The resulting reference was not retained by MicroBitDisplay itself, relying instead on internal storage within NRF52LedMatrix. Since that member is private, the timer could not be explicitly managed or cleaned up. Also, NRF52LedMatrix does not allocate the timer, so it should not be responsible for maintaining it.
In rare (more advanced) scenarios, this could lead to orphaned timers and memory leaks.

Implementation details

• The display refresh timer is now instantiated in MicroBit, similar to all other timers used by Codal.
• MicroBitDisplay receives the timer via reference, making ownership and lifetime explicit.
• The existing MicroBitDisplay constructor is preserved for compatibility but is marked as deprecated to discourage further use.
• No functional behaviour of the display refresh logic is changed.

[finneyj]

Thanks @MannuLambrichts - looks good!