USB: Show USB connection notification in strict mode

USB connection notification only pops up when USB is in some modes like
MTP. If property "persist.sys.usb.config" does not contain those modes,
this notification will never come out.

Always show USB connection notification when strict is enabled, and
settings could enable/disable strict mode.

CRs-Fixed: 650691

Change-Id: Ibece41a17279476aa28b3f45449b39c02f643bbc