USB: add help link config flag from Settings

Note: This cl is copied from the aosp/1783637 since
merge conflict issue.

Move the flag for the indication of the validity of help links to
the framework, as it is required here as well.

Set the visibility of help text in SystemUI for USB contamination
under this newly introduced flag.

Bug: 195448626
Test: adb shell dumpsys usb add-port "matrix" ufp
adb shell dumpsys usb set-contaminant-status "matrix" true

Change-Id: I50b7b447616f6b84fa170091c4e4a9dbf7ce6e45