Change the value of AudioManager#FLAG_FROM_KEY

As suggested by API review, this CL changes the value of FLAG_FROM_KEY
from 1 << 12 to 1 << 16. Also expands Javadoc of the flag as suggested.

Bug: 121325822
Test: make update-api -j / make -j
Change-Id: Ie40c609c717c68a97b01d88f68ce8ff465e3a89e