Change the 'Internet' to 'internet'

In Android P, we need to global change 'Internet' to 'internet'.
This cl updates all the strings.

What's more, for some strings, we change the whole sentences to make
them clearer:
1. wifi_cellular_data_fallback_summary
2. vpn_first_always_on_vpn_message

Bug: 68934232
Test: Manual
Change-Id: I6b6249e78df69d125a7ceec2b46f020c2892a105