AshmemTest: test lseek() and read() on ashmem fds

ashmem has in-kernel handlers for lseek() and read(), but they aren't
currently being tested.

Add tests for various seeks on a buffer containing holes.  If we land
inside data, then check that we read() the expected data at that offset.

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

Bug: 37254818
Change-Id: I96135a8cea2ce99932e3bc97b5254c95ef6b264a
Signed-off-by: Greg Hackmann <ghackmann@google.com>