aconfig: create first implementation of aconfig storage java read api

1, See AconfigStorageReadAPI.java to see java APIs to map storage files
and read flag values. It is using fast native annotation, in theory it
should be faster than regular JNI without much of the overhead.

2, The java api calls into Rust wrapper in srcs/lib.rs, note that
MappedByteBuffer is not copied during JNI. In the rust side
implementation we get the underlying raw pointer and buffer size and
reconstruct a rust slice. However, at current implmentation, the string
input such as package name and flag name are most likely copied. They
are converted from JStirng to JavaStr first without copy, then the
into() call to convert it to Rust string. We could potentially optimize
it to without copy.

3, Add an android_test target to lock down the API behaviors.

Bug: b/321077378
Test: atest -c
Change-Id: I8915fe70e8eb341be563c70f85e19e644e8aa6be