Update text for USB mode selection dialog.

- create separate strings to be used for the USB preference summary.
- reverted the previous changes made to change the verb in the usb
preference title.
- updated the summary text for the usb selection description for charge
only.

Bug: 36234108
Test: builds
Change-Id: I4a57ecd6f9be4b3c10885ac1bbba76ff3df02b84