Clean up DevicePolicyManager CA certificate APIs

For consistency all CA-related methods now require the ComponentName
of a DeviceAdminReceiver as the first parameter. Updated javadoc for
this and added in some more detail about the methods in general as
well.

Created two new utility APIs,one to list all installed CAs and one to
remove all installed (user) CAs. Deleted old hasAnyCaCertsInstalled
method because it is now redundant.

@bug 16488006

Change-Id: I55eec17e01489ab323f8a0e68b11592605a7b740