Sanitize VPN label to prevent HTML injection

This commit will try to sanitize the content of VpnDialog. This
commit creates a function which will try to sanitize the VPN
label, if the sanitized VPN label is different from the original
one, which means the VPN label might contain HTML tag or the VPN
label violates the words restriction(may contain some wording
which will mislead the user). For this kind of case, show the
package name instead of the VPN label to prevent misleading the
user.

The malicious VPN app might be able to add a large number of line
breaks with HTML in order to hide the system-displayed text from
the user in the connection request dialog. Thus, sanitizing the
content of the dialog is needed.

Bug: 204554636
Test: N/A
Change-Id: I8eb890fd2e5797d8d6ab5b12f9c628bc9616081d
Merged-In: I8eb890fd2e5797d8d6ab5b12f9c628bc9616081d