AppSearch/IsolatedStorageService: Sync BUILD flag and SystemProperty

Sync the RELEASE_APPSEARCH_ENABLE_ISOLATED_STORAGE build flag and the
enable_isolated_storage SystemProperty. This ensures the SystemProperty
accurately reflects the build configuration during runtime.

Assigning the SystemProperty in base_system.mk allows other products to
make use of this configuration as necessary.

Test: AppSearch CTS Tests
Bug: 394401678
Flag: com.android.appsearch.flags.enable_isolated_storage
Ignore-AOSP-First: Internal First Project
Change-Id: Id1bc538ea3401b5a5d9f0cf810ed92d4cd06e59f