Adding an easy uninstall for active device admin

Currently the uninstall button is disabled for a package
with an active device admin. This change enables the button,
which when clicked gives the user an option to deactivate
all the DAs in the package and then uninstall the package.

Bug: b/22359208

Change-Id: I8b955305927751185a4c982dadb5b1b6b07efe5e