Build java_sdk_library modules when UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true

java_sdk_library modules are usually disabled in favor of prebuilts
when TARGET_BUILD_APPS is set.  Keep them enabled when
UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true, which is used by some unbundled
builds built from a full source tree like mainline modules.

Bug: 123183568
Test: m TARGET_PRODUCT=mainline_modules_arm UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true TARGET_BUILD_APPS=com.android.media
Change-Id: Ifa940769e467ca4b7ad3b95106e184c00680d2ae