Handle Home shortcut in PhoneWindowManager and back in native

Home: This is fully handled by System and can be handled in
PhoneWindowManager like all the other shortcuts.

Back: This event need to be sent to application for handling. Since,
meta based shortcuts or KeyEvents are not sent to application, we
need to pre-convert Meta+` and Meta+arrow_left to KEYCODE_BACK before
sending to App. This is handled on the native side.

Test: manual
Bug: 270242945
Bug: 270241579
Change-Id: I2b0773f3c3d1623a82740ac67f59a30f4d90d2c8