BufferPool: Guarantee double-loadability

When a library is double loadable, there may be 2 singleton objects in
the same process. Using the pid as an identifier is not sufficient. This
CL adds a bit to the identifier based on which side the library is
loaded.

Test: atest CtsMediaTestCases -- \
--module-arg CtsMediaTestCases:size:small

Bug: 147147992
Change-Id: If3b52e4491ced58ee71bb55f0fc4c3c17569c0f3