Use on-screen text in a11y text changed event

The beforeText in a text changed event was coming from
the underlying CharSequence, not what was on the screen.
As a result, it was not sent for password fields. Now
we just provide whatever is on the screen, which is
obfuscated to the same degree it is for a user looking
at the screen.

Bug: 75324441
Test: Verified with password field in TalkBack sample app.
Also atest CtsAccessibilityServiceTestCases

Change-Id: Ib7a10313d2f9d0e44d2052e81e47f19967a67bc4