Fix apps build

Fix two problems with Soong's aapt2 rules that occur when
TARGET_BUILD_APPS is set.  Fix escaping the values passed to
--min-sdk-version, they are sometimes P-$$(cat out/build_number.txt).
Also use the correct path to the aapt2 prebuilt.

Bug: 69917341
Test: m TARGET_PRODUCT=full TARGET_BUILD_VARIANT=userdebug TARGET_BUILD_APPS=Camera2
Change-Id: I8660a62c4a87a74e0ac1b9d03d67103af93a0cbe