Enable LOCAL_STRIP_MODULE for prebuilt binaries

Currently, prebuilt binaries with debug symbols are installed unstripped.

There are only a few of prebuilt shared library (that I'm trying to fix),
so I can do this manually by making intermediate target and applying
TARGET_STRIP, each of them, one by one.

But dynamic_binary.mk has more features than stripping binaries
(like copying unstripped binaries to symbols directory)
and if I do it manually, they will lose all the benefits.

Note that this doesn't change anything when LOCAL_STRIP_MODULE is not set.
I actually tried to force strip every BUILD_PREBUILT'ed modules,
but there were a few problems:

- Some packages are not installed (i.e. not in PRODUCT_PACAKGES)
  but are built (i.e. in ALL_MODULES).  And some of them are built in spite
  that they do not have appropriate prebuilt shared library for TARGET_ARCH.
  Stripping them causes "unknown format".

- Some prebuilt modules set LOCAL_MODULE_CLASS incorrectly.
  Example is default.supp of external/valgrind/main, which should be ETC,
  not SHARED_LIBRARY.

Both are better fixed, but I concluded that it's better to be conservative.

Bug: 4585734
Change-Id: If71723b1d76007d45b02429ea5161a8265dd5b6d