[Settings] add @Readable back for block_untrusted_touches, maximum_obscuring_opacity_for_touch

These settings are marked @hide to encourage the usage of wrapper APIs.
They should still be readable for any app via the public APIs in
InputManager.

BUG: 183067125
Test: atest android.server.wm.WindowUntrustedTouchTest
Change-Id: I2a3aff0e3bb2ecfba439323fc09b353bd0f2106f