Accessibility: optionally allow Power key to end the current call.

This is part 2 of the fix for bug 2364220 "Accessibility improvements for
ending calls".

This change updates the POWER key logic in interceptKeyTq() to check the
value of Settings.Secure.INCALL_POWER_BUTTON_BEHAVIOR, which allows the
user to specify that the Power button should hang up while in-call,
instead of just turning off the screen.

Bug: 2364220

Change-Id: If6d8e3155f7d60142ab6fd61f0a9db7f0b0d95ab