new buffer reader for buffer read

This change will create new buffer reader when the code needs to read
the buffer read. Previously there is only new buffer reader in the
class, so if there are multiple threads call the get method in
PackageTable, then these threads will modify the buffer position can
cause a race condition.

This change will create new buffer reader when the code needs to read
the buffer in the method. so each thread will have its own instance of
the reader, and the reader will maintain the position.

Test: atest aconfig_storage_file.test.java
Bug: 397997135
Flag: EXAMPT refactor
Change-Id: I31ac2ab661ec1711f817068917915ae63e1e8dd4