Remove original modifiers from generated fallback keys

Previously generated fallbacks would preserve the original modifiers,
leading to unexpected key events.
Example: Ctrl + Space -> Ctrl + language_switch

This change removes the modifiers used to trigger the fallback.
Example: Ctrl + Space -> language_switch

Fallbacks are triggered only with exact matching, so extra
modifiers will not output a fallback.
Example: Ctrl + Alt + Space -> Ctrl + Alt + Space (no fallback)

Bug: 382545048
Test: atest com.android.test.input.KeyCharacterMapTest

Flag: com.android.hardware.input.remove_fallback_modifiers

Change-Id: I78de4bc131b0c9171a3508ee560135fce6b77e42