CursorWindow_test: fix leaks

`buf` isn't freed here. Convert it to either a unique_ptr or stack
array, depending on size.

Caught by the static analyzer:
> frameworks/base/libs/androidfw/tests/CursorWindow_test.cpp:190:5:
warning: Potential leak of memory pointed to by 'buf'
[clang-analyzer-unix.Malloc]
> frameworks/base/libs/androidfw/tests/CursorWindow_test.cpp:268:5:
warning: Potential leak of memory pointed to by 'buf'
[clang-analyzer-unix.Malloc]
> frameworks/base/libs/androidfw/tests/CursorWindow_test.cpp:327:5:
warning: Potential leak of memory pointed to by 'buf'
[clang-analyzer-unix.Malloc]

Bug: 206470603
Test: TreeHugger
Change-Id: I6fee4a9c2eedce0385f794567b0083df24c63419