Update the prefix in keystore for app exclusion

Update the string without the underscore to prevent conflict with
the existing VPN prefix.

Settings app may get exception when the vpn exclusion list was
set since the logic is used in the Settings relying on the
prefix match. The current app exclusion prefix is also started
with "VPN_", so it will be mis-used by Settings as a VPN profile.

Bug: 237345836
Test: atest FrameworksNetTests
Change-Id: I01e773cc15eb1ae5ffaa12aef124bbbf390cc004