Support initializing ProcessState with custom binder window size.

While the binder kernel driver allocates memory lazily, using
a 1MB binder window does mean 1MB of vmalloc address space is lost.
This can be a problem on 32-bit devices, where the address space
available for vmalloc is very limited.

This change allows initializing ProcessState with a custom mmap
size. Note that this API must be called as early as possible in
the process, before any other hwbinder (API) calls or HIDL calls
have been made.

c/p from 32507d340df9c654ae3129ce666c6a2b5d708719

Bug: 129785390
Test: verify kernel output after calling API
Change-Id: I62cdbcb4ffcad1a6d7b3eeae5560eb3e5e602515