Ensure input methods setting keys unreadable to downloaded apps

ENABLED_INPUT_METHODS and DISABLED_SYSTEM_INPUT_METHODS reveals
a list of input methods apps, which violates the rules of package
visibility. This cl restrict the setting key to be readable for
apps targeting in T and below.

Bug: 253164661
Test: atest AccessibilitySoftKeyboardTest
Test: atest CarRotaryImeTest
Change-Id: I48aa7b05cd0dbde70fc6d53eee455959264a6867