Correct method signature of screenshot keychord invocation

Fixes the screenshot call in PhoneWindowManager so that the
screenshot source is correctly set.

The root cause is that Handler.java has method signatures:

obtainMessage(int msg, int arg1, int arg2)
obtainMessage(int msg, Object obj)

As part of the ScreenshotRequest refactor (ag/20894854), arg2 was no longer needed in PhoneWindowManager and was removed. However, this silently changed the signature (setting msg.obj instead of msg.arg1), and wasn't noticed since the int arg1 just gets cast to an object. Ultimately this makes the argument default to 0, which (once the ScreenshotRequest is constructed) gets translated to describing a global actions invocation.

Bug: b/273560369
Fix: b/273560369
Test: manual (invoke a screenshot using the keychord, verify that
we get a SCREENSHOT_REQUESTED_KEYCHORD log)

Change-Id: I6337226ce7d36aed874e3d3a1a6ebc4ff9a00920