Connect FrameRatePowerSavingsBalanced from WindowManager.LayoutParam to

ViewRootImpl

Previously, we have a variable (mIsFrameRatePowerSavingsBalanced) in ViewRootImpl to determine whehter we
want to enable Toolkit dVRR feature. There are two different ways
developers can set this value:
1. Use the style attribute IsFrameRatePowerSavingsBalancedEnabled in
   styles.xml. This value will be picked up in PhoneWindow and assigned
   to VieWRootImpl directly.
2. Use Window to call the setter function - setFrameRatePowerSavingsBalanced to update the LayoutParam attr

When introducing this API, we only check the value from the first
approach but not the second one. In this CL, the information is
consolidated in WindowManager.LayoutParams, and ViewRootImpl will
retrive this value with mWindowAttributes.isFrameRatePowerSavingsBalanced()

Test: atest ViewRootImplTest / atest PhoneWindowTest
Change-Id: If65a07b2f2064020f25cbecd506e993f5f51ee6d