WifiManager: API upates for O

Deprecate the following API's:
1. pingSupplicant() - We don't want apps trying to figure out if
wpa_supplicant is running or not. It should not be something that
apps need to know.
2. saveConfiguration() - No longer needed, we automatically persist data
when needed.

Other changes:
1. Reworded the docstrings to remove direct reference to wpa_supplicant
for network configuration related API's.
2. We no longer allow apps to modfiy network configuration of other
apps. Added a docstring explaining that in the 3 modify API's.
3. Changed the wording of enableNetwork's |disableOthers| flag.

Bug: 31146757
Test: make update-api
Change-Id: If970d85146de3532072191cb2b7ecb0d5adb2774