Update max_sdk_version from SdkSpec to ApiLevel

max_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 max_sdk_version in bp
files

Bug: 208456999
Test: no change in ninja file (this should be a no-op)
Test: TH
Change-Id: I304b5ad802bde200137d8e225182828dfd6f7227