Update target_sdk_version from SdkSpec to ApiLevel

target_sdk_version signifies device version and does not need an sdkKind to
describe it fully. Update the type and cleanup existing usages. As a
side benefit, we also get better error handling since users can no
longer enter something like `public_30` as a valid target_sdk_version in bp
files

Test: m nothing
Test: no change in ninja files (this should be a no-op)
Bug: 208456999

Change-Id: I3c19245e29184bd9e5660ad8981966f64dfa9424