[DocumentsUI] Remove explicit targetSdkVersion

Going forward targetSdkVersion would be set by build system.
It would be by default set to 10000 before SDK finalization,
and updated to the new API level after finalization.
Setting updatable flag as true for DocumentsUI.

Effectively it would mean:
1. '10000' in aosp and internal master
2. Finalized number in development branches like "33" in "tm-dev"
3. As sdk hasn't been finalised in "udc-dev", it would be "10000"
which would be automatically updated to finalized version after sdk finalization.

Removing the target sdk version declaration from the blueprint and manifest files.
More details can be found here go/mainline-modules-target-sdk-version-fix

Bug: b/242296003
Test: croot; sdks=`find ./out -name ‘DocumentsUI.apk' | grep -v .intermediates`; echo $sdks; for sdk in $sdks;do aapt2 dump badging `echo $sdk` | grep targetSdkVersion:; done
1. outputs 33 in tm-mainline-prod
2. outputs 10000 in master

Change-Id: Iaf092fafcaec5108c7c37c42fd15c791c5def9a9