Make mini-debug-info stripping pass on already stripped binaries

Prebuilts may already be stripped.  Instead of requiring every stripped
prebuilt to be annotated with LOCAL_STRIP_MODULE := false, just make
the default (mini-debug-info) pass through already-stripped modules
without failing.

Bug: 36793128
Test: m -j checkbuild
Change-Id: Iafc4614ceb91bbbb2e5ff82caefd18753f8961b1
Merged-In: Iafc4614ceb91bbbb2e5ff82caefd18753f8961b1
(cherry picked from commit 5e93057d)