aflags: read protos from all containers

Create one library for reading protos from all containers, instead of
having numerous libraries perform the same logic. For Java, we will
create a similar library reusing the same
partition_aconfig_flags_paths.txt.

Bug: 324436145
Test: adb shell aflags list # Confirm that various containers appear
Change-Id: I924e281a50f9a609e1c07c03267eebe3dce52752