fs_mgr: move AVB related source into libfs_avb

This CL also adds namespace android::fs_mgr and remove FsManager* prefix
for class names. Note that android::fs_mgr::FsManagerAvbOps will be removed
in later CLs when fs_mgr doesn't rely on libavb->avb_slot_verify() to
parse vbmeta structs.

Some lingering sources for by_name_symlink_map_ are also removed.

Bug: 112103720
Test: boot crosshatch_mainline-userdebug
Change-Id: I2d1a5cc39bcd5a699da8d5539f191d8c7737c4af