Replacing accessibility service permissions with capability attributes.

Accessibility services can perform special operations such as retrieve
the screen content, enable explore by touch, etc. To ensure the user
is aware that the service will perform special operations we were using
permissions. However, the special operations cannot be performed unless
the service is really enabled by the user and it is at this point that
we want to notify the user about the service capabilities.

This change adds capability attributes to the accessibility service's
meta-data XML file. The service has to declare the capability and when
it is enabled we show the user the capabilities in the warining dialog.

bug:8633951

Change-Id: Id3442dc71dad018e606888afdc40834682fdb037