Allow building userdata partition in Soong

The soong vs make generated userdata images are still not bit identical,
but this change resolves execution time failure from build_image when
building userdata.img.

Implementation details:
- Introduce partition_size property to filesystem module
- Specify the correct mount point for userdata partition

Test: m out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_userdata_image/android_common/userdata.img
Bug: 388920173
Change-Id: I2c0945ce70d74c632ba241c8a93c60763cfe87e7