aconfig: update aconfig_storage_read_api

Right now, aconfig_storage_read_api rust lib would own mapped files. So
individual flag lib does not need to. The original intent is that all
these flag lib can dynamically link against this lib. So say we have
different variants of flag lib for one flag package, there is just one
unique memory mapped file. In addition, the exposed rust apis hides the
memory mapped files. So when exporting these apis to c++/java, we don't
have to implement memory mapping in c++/java flag read api lib.

However, this design has its own downside. The biggest being that these
memory mapped files are stored in a mutable static hash map. For rust,
it must be protected by a mutex lock. The real effect on the lock is
mostly minimal as it immediately unlocks when the clone of memory
mapping is returned. But still we could get more performance out of it
in exchange of having more c++/java side implementation of mapping
files. (Note, apis that interpret mapped files are still exported by
Rust, so no c++/java implementation needed).

In this change, aconfig_storage_read_api give away its ownership of
memory mapped file in its api to get memory mapped file. The memory
mapped file is supposed to be owned by generated flag lib. By doing so,
no more mutex lock is needed. Also, since each flag lib has one flag
package, which maps to a single owning container. The flag lib can have
the memory mapped files as non mutable static variables. So no mutex
lock penalty on flag lib side.

Bug: b/321077378
Test: atest aconfig_storage_read_api.test; atest
aconfig_storage_read_api.test.rust

Change-Id: I29060bae90db6f7d296252ed73f7e312143f4b7e