Extend MenuItem to allow modifier keys for shortcut

Before this, CTRL was the default modifier for MenuItem shortcuts.
Now, It's possible to specify the modifier in XML or via MenuItem
methods.
The modifier list includes META, CTRL, ALT, SHIFT, SYM and FUNCTION.

WIP on support library implementation

Bug: 32165848
Test: Run the tests using following commands:
> cts-tradefed run cts-dev -m CtsViewTestCases -t
>   android.view.cts.MenuTest#testPerformShortcut
> cts-tradefed run cts-dev -m CtsViewTestCases -t
>   android.view.cts.MenuInflaterTest#testInflateShortcutModifiersFromXml

Change-Id: I7db4607d4657451934761914efe4c250de109653