Make a CommonPool for host builds

CommonPool uses some APIs that are only available on Android builds for
the setting up of the threads in the constructor. This creates
CommonPoolBase as a superclass of CommonPool to contain a setupThread
method. We provide two implementations of CommonPoolBase, one for
Android which copies the setup performed originally in CommonPool.cpp,
and one version for host which removes the calls to Android specific
APIs.
In addition, this adds a way to unblock the threads in the destructor of
CommonPool, as the destructor would otherwise get stuck if ever called.

Bug: 322360037
Test: libhwui tests
Change-Id: I26d67d36a2d5946c56bde375fd2810b5e395193f