Fix file path separator issue when running cts on windows

The test packages including android.bionic, android.nativemeida.xa,
and android.nativemedia.sl use the host file path separator to find
the guest Android path. On Windows, the separator is "\" not "/",
which causes the adb push failed. This patch uses "/" as the separator
directly to fix the test failures.

Change-Id: Ib751c017cd5752b3ebf0cf783c6e7f0ad0accfc1