Change FileStream test buffer from char* void*

It seems like GoogleTest prints string parameters before checking them.
This can be a problem in FileStream tests since these buffers are not
null terminated.  ASan reports heap-buffer-overflow.

Test: m SANITIZE_HOST=address
out/soong/.intermediates/frameworks/base/tools/aapt2/aapt2_tests/linux_glibc_x86_64_asan/aapt2_tests \
      LD_LIBRARY_PATH=out/soong/.intermediates/external/libcxx/libc++/linux_glibc_x86_64_shared
out/soong/.intermediates/frameworks/base/tools/aapt2/aapt2_tests/linux_glibc_x86_64_asan/aapt2_tests

Change-Id: I9a1956d815729de23c7038fb0d498240eb917070