Add test to check buffer hdr not in user space

Add a test to check that the start of the first
transaction buffer is page aligned and that the
gap between two consecutive buffers is less than
or equal to the number of bytes it takes to align
the first buffer.

Test: run binderLibTest
Bug: 36007193
Change-Id: Ibe8b7437a1d532741b0e002a838264f990848f4e
Signed-off-by: Sherry Yang <sherryy@google.com>