base: Add a key combo to start/stop video recording

Press volume up and power to start recording a video of your screen.
Press this key combo again to stop it.

Change-Id: Id0b13ba1092a1d6e0bf7bff5f14d89764cdf370c

Conflicts:
	packages/SystemUI/res/values/custom_strings.xml
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java