MemoryHeapBase: Map as read-only when needed

When creating a MemoryHeapBase around a file descriptor provided by a
different process, either via an fd or a device name, the existing
code would attempt to map it with PROT_WRITE, unconditionally, which
would result in a failure to map.

With this change, we omit PROT_WRITE from the mapping whenever the
READ_ONLY flag is set, but only when accessing via one of these ctors.
The ctor that allocates a new ashmem region continues to work as
before, with the caller process having write access, but any other
process not having it.

Test: atest -p frameworks/native/libs/binder
Change-Id: Iab3583d841c3dceed1a7cb61e922a85104b4b00b