Cast copied text to string before sending share intent

EditText#getText returns an Editable (editable buffer), not a string.
This is serializable, so it can be put into the intent without error,
but trying to retrieve it with intent.getStringExtra(Intent.EXTRA_TEXT)
returns null. Casting to a string fixes this problem and allows
destination apps to correctly read the copied (and edited) text.

Bug: 227107111
Fix: 227107111
Test: verified that sharing to e.g. Chat copies the text into the
send box (prior to change it would open the app but copy no text).

Change-Id: I3052677ae65556949351e9d112e81d38dc209bc9