Add a backdoor to change FrameTimeline's maxDisplayFrames

This change adds a backdoor entry through SurfaceFlinger to allow
modifying the max number of display frames stored in FrameTimeline. This
is particularly useful for debugging, enabling to take a trace as well
see the dump on the side.

Bug: 169894383
Test: libsurfaceflinger_unittest
Change-Id: I872e240597193d597dac4a9f096b6360edf5cce1