SF: Flag guard setFrameRate() API

- LayerHistory is now always created
- Do not use LayerHistory when content detection flag is turned off.

Bug: 148428554
Test: atest CtsGraphicsTestCases:SetFrameRateTest
Test: Play YouTube video. When content detection flag is off,
      display runs at 90, otherwise at 60.
Test: adb shell /data/.../libsurfaceflinger_unittest
Change-Id: Iab8b79e16a5257c770d9ddd204cfe26d22de83b6