Move Scoped Storage compat flags definition to frameworks/base

Currently the flags definition is in MediaProvider, which is an
apex module (updatable), and the flags values are now used in
frameworks/base as well (non-updatable).

Good to have flags definition in the constant component.

Bug: 150593967
Test: atest ExternalStorageHostTest
Test: atest com.android.providers.media
Test: manual
Change-Id: I0b06c068cb3957a7db8af5e14598724f72122736