SkiaGLRE caching: monitor the caching of shaders

Bug: 178661709
Test: adb shell setprop debug.renderengine.backend skiagl
      adb shell stop && adb shell start
      adb shell dumpsys SurfaceFlinger

Add an implementation of PersistentCache that allows us to monitor
whether a shader was generated when we expected one to be. This can be
repurposed in the future (i.e. once we've generated all the shaders we
expect) to verify that we do *not* generate shaders after primeCache
finishes.

Add the number of recently cached shaders (i.e. since the last call, or
primeCache, if this is the first call) to the output of dumpsys.

Change-Id: I6f177b7ad78fef166d529e918447395629673f47