Fix uninitialized member variable

The default constructor of FileMap was missing an initializer for the
mFileMapping variables. This results in CloseHandle being called with
a "random" value, which can cause havoc in Win32 process over time (e.g.
in the case of libaapt2_jni.dll, which is loaded in a JVM process).

Also, update the code to use "NULL" for invalid file map handle
and "INVALID_HANDLE_VALUE" for invalid file handle.

Bug: 38197857
Test: Stress testing on (Windows) machine
Change-Id: Ibd769219d601fbafcfcee89e848b31cc5137826c