Add documentation to VibratorManager on foreground/background behavior

Add information to javadocs explaining that apps in background can
only vibrate the device if the given AudioAttributes usage is one of
notification, ringtone or alarm usages.

Bug: 146313440
Test: N/A
Change-Id: Ib0b8401539157f0ce2768aab281f3b23aae38dae
1 file changed