Fix one-handed mode tap app to exit doesn't work in factory boot

Root cause:
1. Design change "tap app to exit" -> always being true, and settings
remove this option.
2. Tuning performance, due to 1, no longer listens to the key change,
so we remove the listener.

Solution:
Change getSettingsTapsAppToExit() API to return default TRUE.

Bug: 195629000
Test: Build ROM and factory-reset to test
Test: atest WMShellUnitTests
Change-Id: Id3c265258ed28f7e0aa54505c339671885d28fa7