com_android_a11y_menu_flags is used from system_ext app

As a system_ext app is using the flags, the flags must be in the
system_ext container instead of the system container.

Bug: 385213580
Test: "adb shell aflags list" must not be changed.
Flag: EXEMPT refactor
Change-Id: I37c2807766b85cf3d5b6c9b2f1e43c278755b3a4