16kb: Define PRODUCT_BOOTS_16K to indicate that target boots with 16kb kernel

This build flag will indicate whether the target boots with 16kb kernel.
This flag will be used in pgagnostic targets and targets with
with the _16k suffix.

Define the build flag PRODUCT_BOOTS_16K

- When PRODUCT_BOOTS_16K := true, target boots with 16kb kernel.
- When PRODUCT_BOOTS_16K := false, target boots with 4kb kernel.

If the PRODUCT_BOOTS_16K is not defined, its value will be false by default.

Bug: 346868433
Test: Built Pixel 8 targets
      $ source build/envsetup.sh
      $ lunch husky-trunk_staging-userdebug
      $ get_build_var PRODUCT_BOOTS_16K
      false
Change-Id: Ifc5138dd76d2172e1c3bc2564efe91a0f83c5420