docs: Updated reference docs for changed permission level

Updated reference to note that SYSTEM_ALERT_WINDOW is now "signature"
instead of "dangerous". (The Manifest.permission class and javadoc
is automatically generated from Manifest.xml.)

Also clarified how an app can request SYSTEM_ALERT_WINDOW and
WRITE_SETTINGS privileges (it's different than for ordinary
'dangerous' permissions).

See first comment for doc stage location.

bug: 24673125
Change-Id: Id5908b6cc8c8ea3d6901f245cd35c771bdd2eb87