Fixed ApnEditor issue

The mainly changed:
1. Will not insert invalid apn data to database.
2. Clicking back button will save the APN data to database if the data is valid.

Test: make ROBOTEST_FILTER=ApnEditorTest -j40 RunSettingsRoboTests
Bug: 73745458
Bug: 67327863
Change-Id: Ie2c147cae03ad78d43c351e05add761b2dffac0c