libbinder_ndk: nice dup operator

Common operations for ScopedFileDescriptor. Not clear to need to do
ScopedFileDescriptor(::dup(foo.get())) b/c 'get' method is inherited
here, and justifying duplicated API surface because this method does the
right thing wrt ownership.

Bug: email thread
Test: CtsNdkBinderTestCases
Change-Id: I60a281a1e2ddb42bc0768a0114a5e9e7b22aaed2