[sf] Move texture deletion to main thread via pool

Places deleted texture names into the texture pool, and adds code to
postComposition to either generate textures if the pool is low or delete
them if it is overfilled.

Bug: 129393280
Test: Manual, systrace with gesture nav and observe deferred deletions
Test: libsurfaceflinger_unittest, which was updated to pass
Change-Id: I683ad5b817a29f679def22f0b746223aac3241d9