WindowManager: Add system property to enable navigation bar

New system property (ro.hw.nav_keys) to indicate if device has
hardware navigation keys (home, back, etc.).  If the property
is set to "0" (not present), then display the Android navigation
bar.

Change-Id: I5c1a7b0584bb2d8926f69ae5520f44617c982f07
performance: Add Performance class and PerfLock API.

New Performance class with PerfLock APIs to allow
system level performance optimizations to be toggled.

Change-Id: I9cd080c1ed031f308cafa8e1bb4510c16ac1a09f

Conflicts:
	core/jni/Android.mk
	core/jni/AndroidRuntime.cpp