InputSettings: add setting to disable touchpads

For accessibility reasons, we want users to be able to disable
touchpads. For example, users who are visually impaired may want to
disable the touchpad from a keyboard/touchpad combo device to avoid
accidental clicks or pointer movements, while still using the keyboard.

Due to the potentially disruptive nature of the setting it will not be
backed up and restored. For example, if a user thought that their built
in touchpad was broken and attempted a factory reset to fix it,
restoring the settings would then “break” it again.

This can be tested with the following ADB command:

    $ adb shell settings put system touchpad_enabled 0

(Or the same with 1 to re-enable.)

Test: connect a touchpad, disable touchpads, check it stops working.
      Turn the toggle back on, check it starts working
Test: with touchpads disabled, connect a touchpad, check it doesn't
      work. Turn the toggle back on, check it starts working
Test: with a keyboard/touchpad combo (I tested with the Lenovo P12 Pro's
      detachable unit), check that the keyboard remains usable when the
      touchpad is disabled
Test: $ atest com.android.server.input.InputManagerServiceTests
Test: $ atest SettingsProviderTest
Flag: com.android.hardware.input.touchpad_disable
Bug: 372857036
Bug: 376669241
Change-Id: I1938512872b462e66e4ad822b08614a254280f01