Add sdk mutator for native modules

Compiling native modules against the NDK disables platform features
like ASAN.  For anything shipped on the system image there is no
reason to compile against the NDK.  Add a new mutator to Soong that
creates a platform and an SDK variant for modules that set
sdk_version, and ignore sdk_version for the platform variant.  The
SDK variant will be used for embedding in APKs that may be installed
on older platforms.  Apexes use their own variants that enforce
backwards compatibility.

Test: sdk_test.go
Test: TestJNIPackaging
Bug: 149591340
Change-Id: I7d72934aaee2e1326cc0ba5f29f51f14feec4521