packages/apps/DocumentsUI: Set LOCAL_SDK_VERSION where possible.

This change sets LOCAL_SDK_VERSION for all packages where
this is possible without breaking the build, and
LOCAL_PRIVATE_PLATFORM_APIS := true otherwise.

Setting one of these two will be made required soon, and this
is a change in preparation for that. Not setting LOCAL_SDK_VERSION
makes the app implicitly depend on the bootclasspath, which is
often not required. This change effectively makes depending on
private apis opt-in rather than opt-out.

Test: make relevant packages
Bug: 73535841
Merged-In: Ia8cb832ecb40050a5573959dab15e2deec0fb4c0
Change-Id: Ifbfef4f70eebde919db62a3317f3a412f4352dd5