Don't extract jni from prebuilt apks.

- We don't need LOCAL_PAGE_ALIGN_JNI_SHARED_LIBRARIES now, for we always
  page-align jni shared libraries and store them umcompressed.
- For prebuilt apks, we don't extract jni any more; Instead we always run
  uncompress-shared-libs on them.
- For apks built from source, we still install the jni separately, because
  that way multiple apks can share the same jni and it saves space.

With this change, for most prebuilt apks, we don't need to specify
LOCAL_PREBUILT_JNI_LIBS ("@lib/<abi>/foo.so") any more, for the build
system automatically replaces the embedded jni with uncompressed files;
But if a prebuilt is a fat apk (i.e. containing jni not needed by the
current product architecture), you still need LOCAL_PREBUILT_JNI_LIBS to
specify what jni to keep. Otherwise all embedded jni will be replaced with
uncompressed files, that wastes space.

Bug: 8076853
Change-Id: Ic3666dc72bf17cd293787414dd185470b365f967