android: binder: Add allocator selftest

binder_alloc_selftest tests that alloc_new_buf handles page allocation and
deallocation properly when allocate and free buffers. The test allocates 5
buffers of various sizes to cover all possible page alignment cases, and
frees the buffers using a list of exhaustive freeing order.

Test: boot the device with ANDROID_BINDER_IPC_SELFTEST config option
enabled. Allocator selftest passes.

Bug: 36007193
Change-Id: If21dd645ff0fec4bcfc12f1254b4d9b7520ce936
Signed-off-by: Sherry Yang <sherryy@android.com>