Add a TexturePool class into layer caching

In order to reduce the risk of buffer allocations burning cycles during
rendering, add a texture pool which preallocates a set of screen-sized
buffers available for use for layer caching.

This texture pool has some properties to ease implementation details:
1. Textures are allocated synchronously. It's not very hard to add
asynchronous support, but that does add a bit of complexity and Android
12 is almost finalized. It's also not yet clear if asynchronous
allocation is needed since we can just tune the min texture pool size so
that more buffers are preallocated; although it is certainly more
flexible if we're able to defer allocations off-thread.
2. The texture pool is soft-bounded. If needed additional textures may
be allocated to allow for complex geometries, but beyond an upper bound
any additional textures may be deallocated.
3. The texture pool only supports screen-sized buffers, so we don't
allocate smaller scratch buffers to save on memory. However, typically
layer caching only caches screen-sized buffers. This also means that if
the display size changes, then the texture pool must be reallocated.
Note that the "display size" really refers to the framebuffer size, not
necessarily the app-visible display.

Bug: 184860700
Test: builds, boots
Test: libcompositionengine_test
Change-Id: Ic084310bae2b41be49c2cfe9f8c5a20ff683d1ad