docs: Removed READ_EXTERNAL_STORAGE from 'normal' permissions

Per communication from Ben, READ_EXTERNAL_STORAGE was incorrectly
marked as a 'normal' permission in preview 1, and is properly
changed to 'dangerous' for preview 2. Updating the docs accordingly.

See first comment for doc stage location.

Change-Id: I0f824fb9689d2e7323e505b0200fda23eaa71eed