PRODUCT_PACKAGES: change all places to be appended

with the "+=" operation, instead of the overwriting
operation of ":=", to avoid unexpected build errors
when inherit-product from other files.

Bug: 265084621
Test: build db845c successfully

Change-Id: Id0e7e444af2069b34bb36245163aac9799e6e3d1
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
3 files changed