Update min_sdk_version from SdkSpec to ApiLevel

min_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 min_sdk_version in bp
files

Will do a similar cleanup for targetSdkVersion and maxSdkVersion in a
followup CL

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

Change-Id: Ie6ae7e267d093c5e4787e82685daaca1021d202e