Introduce modifier shortcut key unit tests

This includes these tests for phone device:
Shortcut will start an intent to launch the specific app by category:
    Meta + A         - launch calculator
    Meta + B         - launch browser
    Meta + C         - launch contacts
    Meta + E         - launch email
    Meta + L         - launch calendar
    Meta + M         - launch maps
    Meta + P         - launch music
    Meta + S         - launch messaging

Shortcut will trigger specific behavior:
    Alt + Tab        - show recent apps
    Ctrl + Space     - switch keyboard layout
    Meta + Space     - switch keyboard layout
    Ctrl + Alt + Z   - enable accessibility service
    Meta + Ctrl + S  - take screenshot
    Meta + N         - expand notification panel
    Meta + Slash     - toggle shortcuts menu
    Meta + Alt       - toggle cap lock

Bug: 193278327
Test: atest ModifierShortcutTests

Change-Id: I713a0cccdf7304e84fa8df71cfd4c4d51305c887