Backlight OCR: Correctly handles changes to the accessibility state

When the Media App (AKA Backlight) is started while ChromeVox
or Select-To-Speak are off, any accessibility trees constructed will not
be received by any accessibility service.
The same bug occurs if while viewing a PDF, the ChromeVox user
turns CV off temporarily. Upon turning it back on, the PDF
will appear as empty.
So, when the assistive software is turned on we need
to resend any trees that had already been unsuccessfully sent.
Also, during the period when accessibility is turned off, we need
to release the memory used by the OCR Service, because it could be several MB.

Also took the opportunity:
1. Rename SetPdfOcrEnabledState to ToggleIsAccessibilityState because
that's what actually happens.
2. Remove the use of private inheritance because it's disallowed by
Google's C++ style guide.
3. Make some methods that were unnecessarily public private.

AX-Relnotes: Accessibility now works if ChromeVox is turned on after Media App is launched.
Change-Id: I1f87728f0665ebdc4d08c7e0bf9e3ba7b0364fa1
Fixed: 340953051
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5675351
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1363236}