SF: Add GL texture pool

Adds a pool of GL texture names.

Prior to this change, Layer creation was forced to run on the SF main
thread because it would need to call into RenderEngine to generate a new
texture name.

By creating a pool of pre-generated texture names, this operation no
longer needs to run on the main thread, which unblocks the rest of the
system during operations such as fingerprint unlock.

Bug: 110477323
Test: SurfaceFlinger_test + manual: examine systrace and observe that
      layer creation no longer blocks on access to the main thread

Change-Id: I9d68874d6c6f704c8884676454e84d916cd86507
Merged-In: I9d68874d6c6f704c8884676454e84d916cd86507