Flag to control if generated SDK prebuilts are preferred.

This is needed because:
- the SDK prebuilt drop is being automated, including the Android.bp
- We want to control the prefer flag on a per-module level

It augments the existing SOONG_SDK_SNAPSHOT_PREFER which now overrides
this new flag when set to true.

Bug: 188427719
Test: m nothing
Change-Id: Ieb5ab9fab53a34c615345b7a9d19cadf713eec26