MemoryFile constructor and native methods throw IOExceptions.

These native methods in android.os.MemoryFile throw IOException but their
Java declarations did not include "throws IOException":
native_open(),native_mmap(),native_read(),native_write(),native_pin()

The MemoryFile(String,int) constructor calls native_open and
native_mmap, but does not declare that it throws IOException. The other
Java methods that call the native methods do actually declare that they
throw IOException.

This means that any code that created memory files could throw
an IOException, without knowing about it.

This changes adds "throws IOException" to the native methods and to
the constructor. The constructor change changes the public API, but
maintains binary compatibility. There is some precedent for making
source incompatible source API changes for this sort of thing
(see https://mondrian.corp.google.com/changelist/124214-p9).

The change also makes the native methods static, which
they seem to have been intended to be, as indicated by the
second parameter to the native implementations being named
"clazz".

This requires changes to the Compatibility Test Suite to catch the exceptions.
This is done in https://android-git.corp.google.com/g/2617
Unfortunately that change must be submitted together with this one in order
not to break the build.

Fixes http://b/issue?id=1881829