surfaceflinger: fix missing pure virtual declarations.

android::frametimeline::FrameTimeline forgot to declare a pair of
methods as pure virtual, which results in the following link failure
when building at -O0 because the compiler expects to be able to only
emit the class's vtable in the translation unit that defines the first
non-pure method, which doesn't exist:

  ld.lld: error: undefined symbol: vtable for android::frametimeline::FrameTimeline
  >>> referenced by FrameTimeline.h:279 (frameworks/native/services/surfaceflinger/FrameTimeline/FrameTimeline.h:279)
  >>>               lto.tmp:(android::frametimeline::FrameTimeline::FrameTimeline())
  >>> referenced by FrameTimeline.h:279 (frameworks/native/services/surfaceflinger/FrameTimeline/FrameTimeline.h:279)
  >>>               lto.tmp:(android::frametimeline::FrameTimeline::FrameTimeline())

Test: `mmma frameworks/native/surfaceflinger` after adding -O0 to surfaceflinger_defaults
Change-Id: I2c93986ac5add2c603f385154c07c6238d03c663