Add Alt+Shift layout switch, layout switch toast toggle

[rkarabut]

Adds the option to use Alt+Shift keyboard shortcut (in addition to Ctrl-Space) for cycling through keyboard layouts, along with an option to disable toast notifications on layout switch (as they tend to cover input at the bottom of the screen, especially with status bar off). Alt+Shift is more ergonomic than Ctrl-Space for Titan 1/2/Slim, and used to be my preferred option with other keyboards allowing physical key layout switching.

Tested and working on Unihertz Titan 2.

I added the options to the Customization page, please tell me if there is a better place for them.

[rkarabut]

Pulled signing skip fixes from #157

[pzauner]

your "signing skip fixes from #157" were bad practice.

a852de4

[pzauner]

I dropped the signing-skip changes from this PR. They were out of scope and not equivalent to the implementation already on main, especially around stable release validation.

I rebuilt the PR on top of current main as a focused Alt+Shift layout-switch change. The new settings now live in KeyboardLayoutSettingsScreen, alongside the existing layout/profile controls, instead of the generic customization screen. I also kept the toast toggle wired into the existing Ctrl+Space subtype switch path.

I added targeted tests for the new SettingsManager preferences, backup/restore schema coverage, and the IME Alt+Shift shortcut behavior to make sure the shortcut is consumed and modifier state is cleaned up.

[rkarabut]

Thanks, should've clarified it was to see if CI passes and to be reverted

your "signing skip fixes from #157" were bad practice.

a852de4