AshmemTest: expand ProtTest cases

ashmem buffers start with PROT_EXEC | PROT_READ | PROT_WRITE and can
have bits individually removed (but not added) through the
ASHMEM_SET_PROT_MASK ioctl.  Test that removing prot bits more than once
works, and that the kernel blocks adding prot bits.

Also test that the complementary ASHMEM_GET_PROT_MASK ioctl returns the
expected prot mask.

Test:  /data/nativetest64/libcutils_test/libcutils_test64 \
       --gtest_filter=AshmemTest.* (on hikey)
Test:  /data/nativetest/libcutils_test/libcutils_test32 \
       --gtest_filter=AshmemTest.* (on hikey)

Change-Id: If7b13672547ec4cf6dfd1886197f40f12b2f1c85
Signed-off-by: Greg Hackmann <ghackmann@google.com>