Add device_aconfig_declarations target

Building device_aconfig_declarations creates simplifies listing all
aconfig flags used on the device by creating a build artifact at:
$(OUT_DIR)/target/product/$(TARGET_PRODUCT)/device_aconfig_declarations.pb

Bug: b/308625757
Test: manual
Change-Id: Ic5eef9c6a26f7893b3f4bdfed2cd4ac82a54a8f9