Add frame rate flexibility token

Add support for temporarily relaxing frame rate restrictions in surface
flinger. This is used by CTS tests to get a consistent device state
while running frame rate tests.

Bug: 148033900

Test: - On a Pixel 4, I turned the brightness down and covered the
ambient light sensor, causing the display manager to set a frame rate
restriction. I ran the frame rate CTS test without these CLs applied,
and confirmed the test failed because surface flinger couldn't switch
frame rates, as expected. Then I ran the tests with the CLs applied, and
confirmed the tests pass.

- I confirmed that, without adopting shell permission identity, the CTS
test is denied the request to acquire a frame rate flexibility token. So
normal apps won't be able to access this.

Change-Id: Ie2b611cf5726c14a7a22e315a85bf6200d190682