Change function to check MODULE_BUILD_FROM_SOURCE

com.google.android.conscrypt is not valid for this
usage as S-OS can have a compressed target by default
(e.g com.google.android.conscrypt_compressed)

Same change applied to com.google.android.art check

Also modify to check `true` value of the MODULE_BUILD_FROM_SOURCE
to filter out both unset variable and `false`

Test: adding one error code temporally, install prebuilt
  train, and check result with,without this fix

    MODULE_BUILD_FROM_SOURCE := true
  + $(error we are here, true $(PRODUCT_PACKAGES))
  endif

Bug: 197282276
Change-Id: I864ba1bb5da61ccc6b8576029ab6c569959b2c93